It looked great to me, more than enough for 12 roosters. How many feeders and waterers are you planning to have in it?
Adding new roosters is absolutely doable, though sometimes you have to keep them in a smaller pen within the bigger pen so they can get to know the other roosters a bit before introducing them.
Almost all of my chickens are rescues; it's very rewarding.

If you do end up rescuing some roosters, just remember to quarantine them for a month or two. I've had several that came infested with mites, which was rather unpleasant to treat. Also, always have a backup plan for roosters who don't do well with your rooster flock, especially if the rooster was previously a bully to other roosters.
A cockerel is a fully matured rooster at 1 year old. Usually, though, you can tell a bit earlier whether he'll be good with hens. Since a lot of your chickens are bantams(?) you should probably be fine for a bit, but I would try to build that run as soon as possible. 12 cockerels is quite a lot for 10 pullets.