The grey one is a boy, as is the red one. I don't think he is pure RIR though.. Way too light.. The silkie is a rooster, but I love silkie roos.. You should keep him if you are allowed boys. 10 x 10 is big enough for 25 chickens if you count 4 SF per chicken, but I would go for more like 15. The black one is a girl, and the one with the crest may be as well. I can't say 100% for sure, as the picture doesn't show it up close.
This is my RIR hen. Notice how she is nearly black? The colour should be deep burgundy.

