You could either separate or keep with the flock. For me, it would be as much about behavior issues as gaining weight. Too many cockerels is why folks create/use bachelor pads.
Most folks recommend to butcher a dual purpose cockerel at 4-6 months if you want it to be tender, but others are perfectly happy with year old birds or spent hens, it's all about how you plan to cook them. Low heat, slow moist methods for older birds. I recently cooked a year old Production Red rooster (similar to a black star in that they're bred for high egg laying) in a pressure cooker with chicken soup veggies and some water, and he was delicious and super tender! You can eat any chicken, regardless of size. [People raise and eat quail, and those are tiny.] Age and cooking method determine whether it's tough, tender, and degree of palatability.
I hatched out 20 in January, and only have 4 confirmed hens. No way am I finding a home for ~15 cockerels. I have them in a bachelor pad, and could either feed them meat bird food or all flock food. I've been feeding them 18% all-flock because I got it on sale. By 4-6 months I don't expect more than 3.5 lbs or so dressed weight. Their sire was about that at a year old. How you pen and feed them is all about your goals, really. Raising ~15 production red mix cockerels to butchering age is more expensive than growing out cornish cross chicks, but it didn't seem right to euthanize the roosters at 3 weeks. So they'll get 4-6 good months and one bad day, and I'll get some delicious meat in my freezer.