I think you are serious, they are all the same species, they can and will breed freely. Think of this like dogs or cats- if you have pomeranians and poodles together- they will not stay with their own- they are still all dogs.
if you want them separate to keep the roosters from squabbling or to get breed pure eggs to hatch- then you need to separate them. 12x12 feet for 20 birds? is this just their indoor sleeping quarters, or their entire area? If this is their entire area- this is too small. By the book (ie commercial standard)- they need 3square feet per bird, which you definitely have- but 3 roosters in the mix changes everything- likely they will fight and injure each other (or kill). Also 20 birds will turn this into a dirt/mud pit rapidly. 4-6 hens and a single rooster in a 12x12 pen sounds better to me- especially if they can get some free ranging in the garden/grass ect for a few hours a day on top of their 12x12 pen.
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