Question 1. Is it okay to continue to give them this feed until it is gone or should I try and return it (or donate it) and switch them to grower feed or can I mix them both so the starter feed doesn't go to waste? Also, is there a brand of food that is high quality that I should be looking for?
A: You will likely get a variety of responses here, but here's mine. Generally speaking, the target protein range for ages 1-8 weeks for chicks intended for egg layers is around 20-22%, but you've made it this far on an 18% feed so I'd stick with that. The feed you have is both a starter and a grower feed. At age 9-20ish weeks, 16-18% is the normal range for protein, and you want a grower or all flock feed so they aren't consuming unnecessary amounts of protein. Basically, you can keep using the feed you have until they are close to 20 weeks old. Historically I always switched to layer feed around 17-18 weeks myself so I can integrate them into the flock. That brings me to another point though, since I have been allowing hens to hatch chicks, it's complicated the feeding for my chicks and decided to switch to an all-flock feed and simply provide calcium in the form of egg shells and oyster shells as a free-choice item in another container. If you know you won't be mixing ages/flocks in the future, layer or all-flock at an older age is fine.
Question 2. I moved my flock to the coop a couple of weeks ago (I am in southern California) with a heat lamp in the brooder box at night. I have been putting my chicks up there at night and when I do, I put their food and water in there with them. Should I be doing this?
A: No need to leave food and water in there with them at night as they are sleeping and won't need it, just don't leave the food in a location that it will attract predators or rodents. I leave my feed in the coop to deter rodents, and leave the water out of the coop to keep the bedding dry (personal choice). Your coop looks pretty secure, so I'd just leave it out. No need for a heat lamp at that age unless it's dipping below 45/50 degrees at night. You just don't want to go from cozy extra warm coop one night to super cold, just like humans acclimate to winter temps, birds need to acclimate too.
Question 3. Do roosters need a different diet?
A: This circles back to question one. Many folks historically fed layer feed to the entire flock, roo included, but in theory this is not kind to his kidneys due to excessive calcium. This can be fixed by using an all flock feed and providing egg shells or oyster shells as a free choice option in a separate container for the hens. This is a personal choice, really. My roo is 4 and has lived his whole life thus far on layer feed.
Question 4: What do you recommend as treats? Are there some trees that are poisonous to my sweet chicks?
A: I'm not from your area and therefore not accustomed to your flora, but generally speaking they can eat anything we can except avocado pits and peels, green/raw potatoes and peels, tomato leaves/stems, sugar/candy/chocolate, citrus (affects calcium absorption but not actually poisonous), dry beans, junk food (white bread, chips, fatty foods, etc), or anything moldy or rotten.
Some folks share the thought that because chicken feed is so scientifically formulated, there is no need for treats, and that is true to some extent, but treats are still a lot of fun and a great way to train your flock. I give my chickens veggie scraps, organic scratch grains, and mealworms. Just like with humans, treats should be in moderation. If it gets cold at night, some scratch grains before bed is a good way to keep them warm as digesting warms their bodies.
If they are not getting to free range or have access to dirt, they also need to be given free choice grit once you decide to give them treats. Grit is necessary for digesting food, but a chicken exclusively fed chicken feed would not need grit. When providing grit for the first time to chicks, monitor their intake because they are still learning and may try to eat it like food. Let them eat a little then take it away, at least until they learn some moderation.