I know for a fact that it is impossible to sex them at that age... I researched for ever and tried. I do that if raised as fry or smaller, they should do well with each other in small groups. I would not add any after a few inches... they will single that one out, unless you add a female or alpha male after they reach breeding age. They only trouble with them is they get so big so slowly that they will be out grown and get picked on at times or the reverse when they grow they have been known to eat smaller fish... My 5 incher ate a brevis a few months back... Larger ones have eaten larger fish than shellies... be sure to research the tank mates as well. If you want cichlids, go with larger tangs, or peacocks will go well...