Update: Looking at a permanent coop to upgrade to in a week or so. The one I've found has a 6' x 3.5' run and a 2' x 3.5' coop, so it should be big enough. It's also a prefab coop, but it seems a little better built/sturdier. I will need to install a predator apron and a 2x4 roosting bar, but then it'll be habitable for chickens. I haven't gotten the coop yet, but I will pretty soon.
The chickens look alright for now, but they definitely seem cramped. I've been letting them out into a makeshift wire pen when I can watch them, which seems to be keeping the whole thing afloat. It's also a tractor, so at least they get fresh grass daily. I may have given the impression that the coop wasn't very secure, but it's fairly sturdy. Based on scat I've found, a pretty big dog has been about, but it wasn't able to get in.
I'll send the chickens/supplies/rental coop back as planned in the fall, and get 3 - 4 bantams/silkies for the new coop in the spring. Thanks so much for the help everyone!