1.) Can I safely put my 8-10 week birds in the coop in those temps without having a heat lamp?
Yes, with a few provisos. Once they feather out chickens can handle those temperature very well provided they have a place out of the wind. You often see us talk about draft protection, but we are not talking about the gentle type of draft that comes in around a poorly sealed door or window on the house. Chickens, just like the wild birds you see out in winter, keep themselves warm by trapping tiny pockets of air in their feathers and down. If they get hit by a breeze strong enough to ruffle their feathers those air bubbles can escape and they can get cold. You generally don’t see wild birds out when there is a strong cold wind. They find sheltered places to stay until the wind dies down. I’ve seen feral chickens sleep in trees in temperatures below zero degrees Fahrenheit. They are not on a dead tree branch overlooking a bluff, squawking defiantly in the teeth of a blizzard no matter what Disney shows. They are in a sheltered area out of the wind.
When will chicks of your breeds feather out? That depends on a few things. One is diet. The higher the protein the faster they feather out. That’s a big why Starter is generally around 20% protein while Grower is around 16%. By the time they switch to Starter they are already feathered out.
Another factor is exposure to cold. Even short exposure to colder temperatures help them feather out faster. My 3’ x 6’ brooder is in the coop since I have electricity down there. I put chicks straight from the incubator into that brooder, even when the outside temperature is below freezing. From your screen name I’d guess you have a pretty good idea what my temperatures have been like the last month. I have eleven 4-week-old chicks in that brooder right now. When it is really cold at night I wrap the brooder in plastic to retain heat, the sides are hardware cloth for good ventilation in the summer. But in the winter the area that I heat is pretty toasty. I still sometimes find ice in the far end. I do allow that far end to cool down. The chicks are really good at self-regulating their heat requirements when given a chance.
In my current set-up I have draft protection on the bottom 6” to 8” of that brooder with the sides really open. As you know we’ve had some pretty big temperature swings the last month. When it is warm the chicks move away from the heat. When it is cold, they stick pretty close to the heat source.
I’ve had chicks raised this way go through nights with a low below freezing (mid 20’s) at about 5-1/2 weeks old. They had no problems so I’m being plenty conservative. Since you are raising them in the tropics in your utility room I’d probably wait another week to put them out provided they have breeze protection. In normal conditions chicks of those breeds should feather out between four to five weeks of age but temper that a bit with harsh outside conditions.
2.) Of the available breeds, are any of them better choices because of having to put them into an unheated coop so late in the year?
No. It won’t matter. If you were getting some of the decorative breeds instead of production breeds, maybe, but I don’t have experience with those.
3.) If no breed is necessarily a better choice, would I be well to mix 'n match? And, if so, how many of each...e.g. 5 of each breed, 7 ISA Brown & 8 Black Australorp or whatever?
You can mix ‘n match in any numbers you want. You’ll find some people that say they have had trouble with a specific breed. You’ll find plenty of other people that say they have had absolutely no problem with that specific breed. This includes Rhode Island Reds. Check the posts above. There are different reasons for that. You have to have enough birds of a specific breed for averages to mean anything. Each chicken is an individual. If you have just a few of a specific breed, just one chicken can throw the averages off.
We all have our favorite breeds or sometimes breeds we don’t like. That can be based on any experiences we’ve had or just personal preferences. In my opinion, and it is just my opinion, if you are nervous about RIR’s don’t get them. If you have other options then you can consider them. You can also have a great flock with BA’s and ISA browns.
Also behaviors are partially inherited. If the person selecting which birds are allowed to mate consider non-aggressive behaviors when they select breeding chickens, after a few generations you have a flock of pretty non-aggressive birds. If the person selecting the breeders don’t have that as a factor then there is no telling what behaviors you might get. As a general rule, hatcheries don’t consider behaviors when selecting their breeding birds. A lot of breeders don’t either.
4.) As a newbie, am I going to be able to handle 15 birds or am I already a victim of "chicken math" and courting disaster?
One difference in raising five chickens or fifteen is that they grow really fast. They require more space if you have more chicks. The other difference is that you will have more poop to manage. Poop management is pretty important in a brooder or in a coop and run. Dry poop isn’t that bad but wet poop stinks and can harbor diseases. If the brooder is wet from spilled drinking water or their poop builds up enough that it just doesn’t dry out, well you have to work harder to take care of that.
Another potential issue is that chicks create a lot of dust. Some of that is dander, they shed flecks of skin and down. Some of it comes from scratching bedding. If their poop dries out that makes dust too when they scratch it. Some people are allergic to some of that. In any case that dust will get everywhere. Trust me, if you keep them inside for 8 to 10 weeks you will be ready to move them outside.
Lots of people raise chicks in those totes, especially in climate controlled areas. One danger is that you can easily get the chicks too hot. If you brood them outside with fluctuating temperatures keeping them in those totes can get challenging. You can manage that inside much better. As you can see, there are trade-offs in everything.
I don’t know what outbuildings you might have with electricity. Maybe you have a garage, detached or attached. Maybe a workshop or storage building. Maybe even a back porch but watch predator issues there. If you can build a large brooder that provides really good breeze protection and holds the heat in pretty well, heat one end to where it will be toasty and stay toasty even below freezing but give them plenty of room to get away from the heat at those warmer times, you may be a lot happier than trying to brood them in the house. Remember that in or out of the house, dogs or cats might be predators.
Instead of using a heat lamp, something that is all the rage now is to build a tunnel big enough for the chicks to get under when they are older using a heating pad on top instead of using a heat lamp. If I remember right Blooey is pretty high on this method. You might want to PM her for suggestions. She’s really helpful.
Ventilation in the coop is very important, even in cold weather. There are a couple of reasons for his. When their poop breaks down it creates Ammonia. Chickens like most birds have delicate respiratory systems. If the ammonia builds up it can cause serious, even fatal, problems. But ammonia is lighter than air. If you have even a small hole above the chickens’ heads when they are roosting gravity will force the ammonia out and let good air in. It really doesn’t take much of a hole for ammonia to escape because of gravity
The other issue is moisture. It won’t be an issue in a brooder because you are keeping it warm. But high moisture and temperatures below freezing can lead to frostbite. Moisture comes from their poop and from their breathing. You need a fair amount of ventilation up over their heads in winter so any breezes are above them. You don’t want a cold breeze hitting them directly on the roosts in winter because that can release the tiny air bubbles trapped in their down and feathers that keep them warm. Ridge vents it you have a peaked roof can move a lot of air but might get blocked by snow in some climates. It’s generally not a problem here. Gable vents, roof vents, and even a cupola if you want to get fancy will move a lot of air. Leaving the top of your walls open under an overhang (cover with something for predator protection) will also move a lot of air and the overhang will keep most of the rain out. I don’t know how high your windows are or if you can open them from the top but they may not be high enough.
In the summer you can open those windows wide open as long as you have something there like shutters or hardware cloth to keep predators out. You can have vents down low, especially in the shaded cooler side, to help with air exchange. But in the winter be careful of breezes hitting them.