You should wait until the younger ones are the same height(around 14 weeks), unless you can have them near each other but a barrier like chicken wire separating them for a couple of weeks. Then free range mixing works best. Gives the new ones a chance to run away. For instance. I had my 3 newest...
Rooster is a word to describe all male chickens. Cockerel is a rooster less than a year old, Cock is a rooster over a year old. And no your Ameraucana will have a different body type and colored black and white. My rooster is Blue/Black/Splash.
Since I assume you are referring to the black and white one? I am pretty confident now that one is a cockerel. In the next couple of weeks his comb will really start to grow 3 defined rows of peas and be real red. Of course you can't be 100% sure until crow or egg, but that looks like a boy.
He is a Splash Orpington, so your Buff Orp if it is a cockerel will feather similar to mine. Mine is 50% English bloodline which is why he has so much lower fluff. If you post pics of your BO standing(close-up full body side shot) and clear close-up of comb I could take a guess. Also state what...
Hackle feathers are the feathers around the neck. Saddle feathers are the feathers just before the tail(hence the term saddle, just like a horse's saddle) will be long and pointy on cockerels. Here is mine, you can see the long pointy hackle and saddle feathers.
Hens will have saddle feathers...