Yes, the buff is definitely a cockerel. What's really nice about the Japanese is that it's very easy to tell the gender of the chicks a few days after hatching. You're grays are darling! Though one of them looks like it's going to be what I call a naked gray, where they're bred from grays but come out with little to no lacing at all (they're still good for breeding though.)
It's a little tough to provide guidance for breeding when they're so young. The bird you think will be a winner the one day, the next it will be the worse specimen you have. They change a lot in their first three to four months.
Right now, I would look at length of leg and their body posture. How do their wings droop? How is their 'v' shape? Usually that doesn't change to drastically once it's established. Also, in the grays you want to look for crispness of lacing on the cape (both hens and cocks), as well as the saddle feathers on the cocks (they should be an almost identical match to the cape). If you have a bird that's referred to as "leggy", which means that you can see their legs when they're standing (their legs should be short enough to be covered by the wings when looking at their profile), probably not the best bird to keep around, unless you're looking to avoid the lethal gene altogether when breeding.
Also keep in mind to look at their tails, and how their feathers fan out. It's not uncommon to see pinched tails, or tails that don't fan out completely.
Your birds don't look crossed, but again it can be a little hard to tell. Hope they work out for you!