We currently have 20 standard egglayers in an 8 x 10 shed with a run of 60 x 30. It is very spacious for them, plenty of room. We were very thoughtful when planning the interior of the coop (nesting boxes protected by poop boards but fill up space under roost, food and water is piped into the feeders in the coop but bulk storage is outside, ...) so that most of the space is open for the birds to enjoy. I can fit an entire 3-string bale of straw in the center of the coop for them to play in. However, there is rarely a day when our birds cannot go outside due to weather and, we live in a very dry climate. I will admit that even with the dry climate I only do the deep litter method to an extent. I still have a poop board under the roosts and clear out most of the manure every morning. When we had a few weeks of heavy snow, and I gave up hauling the bucket of manure to the compost pile that I couldn't find, that coop really smelled like ammonia...plenty of ventilation but I got a taste of life with humidity in the form of snow.
Since you are much more humid I would stick with stated indoor measurements and more than plenty ventilation, but you could probably have some leeway overall since your birds can be outside almost every day. I'm guessing they would only be confined if there was a hurricane.
Another thing to consider when wondering how many birds to have is the logistics...how many 5-gallon buckets of manure are you willing to haul, where are you going to dump the manure, how close is a hose for drinking water and cleaning, how many water jugs can you carry, how much food does the feeder hold, how often do you have to fill the feeders, how far is the feed store, whre will you store feed bags...For instance, if I plan correctly I can haul enough feed, bedding, and general chicken supplies to last 10-12 weeks. The feed store is not close (25 miles one way) and in bad weather I may not be able to get there for a couple of weeks straight. So if I stock up just before winter (mid-Dec) I can almost make it to Spring if we have a series of un-ending storms...like this year!
So I would encourage that while you think about space you also consider logistics.
And as far as your measurements for your space...8 x 16 coop = plenty of space (if the birds can be outside almost EVERY day)=30 for me...run 8 x 24 would be REALLY pushing it hard for 30 birds, I would have a difficult time with 20 in that space...but I believe in giving them lots of stimulating environment like bushes, trees, plants, pools with fresh water, shade/sun, places to hide, dust baths...I have the luxury of acreage though too. In your case I wouldn't start with more than 15-16 and see how it goes.
Friend...8 x 8 coop = 16 birds ....and 8 x 16 run = 12 birds...I'd stick with 10 birds and see how it goes for a while. I started with a 4 x 8 run and 6 Rhode Island Red hens. It was tight. Twice the run space but twice the birds would still be tight.
And I don't think either of you should consider heavy breeds. Stick with standards or bantams.