Hi Shirley,
Firstly, I don't keep Cochins or Brahmas - so I'm not an expert on them by any means.
The Buff Cochin cockerel should have a larger comb and wattles than the hens - being a larger fowl, they are slow to mature, much like some of the orpingtons and will take 24-26 weeks to mature fully.
It should slowly become clearer which one the male is and he should of course given time start to crow. He may be attempting a small crow already from time to time and if you see this, you can then ring him do you know who he is.
With regard to mixing Cockerels, it is a difficult question. Very few breeds will mix easily (unlike the Kraiencoppe which strangley can have several cocks in a pen).
Usually though if male birds are brought up together (and are not from a game / fighting breed), have enough space and enough hens, there is a chance they will tolerate one another but may have the odd tussle. In the spring, things may get more difficult though as they start to compete.
Then again, sometimes, cockerels will not get on at all.... It's 'suck it and see' really.
Here is a
photo of a male Buff Cochin that won Best Cochin at the National a couple of years ago.
I hope this helps...
Tim