Out of 6 black tailed white japanese we ended up with 2 pullets and the rest are cockerels. We only want to keep the best looking roo. They are 6 weeks old now. I can post pictures if I need to.
6 weeks is pretty young to be making culling decisions unless there are obvious disqualifications or serious defects. Also, I'd advise you to keep 2. If you keep 2 they'll both be fine. If you keep only 1 he'll die & then what will you do?
i have black tailed white japanese as well, at 6 months i am just starting to see what their tails are going to look like as adults, i have 3 roos (of 8) and can tell #1 has a squirrel tail (not good), #2s tail is looking quite nice (aside from not yet having the few very long tail feathers) and #3 has moulted his tail so we shall see in a few weeks what comes back in!
i couldnt have told any younger which was nicest (never had young japs before though), im leaning towards roo #2, well see.
at 6 months i still have them all together and have no problems at all (the 3 jap boys are with the 5 jap hens in their pen, and in another pen i have about 10 cockerals and 12 pullets with no problems), from what i have heard my (first time in many years rasing from chicks) problems dont generally arise until around the 1 year mark when the roosters start fighting for dominance (and for the hens!) but in some breeds people house multiple roosters in small flocks without problems (this can work if you have rasied them together since they were young).