2 females would most likely be fine together.
boy and girl means the girl will get harrassed non stop for loving (once they reach puberty), which could result in bits being bitten off, getting gravid way too young and stress
male with male would probably result in 1 killing the other, either thru fighting or stress.
if u get hatchies they will be ok together fro a few months (if theyre well fed they shouldnt eat each others toes or tails but might) when they hit puberty they may need seperating though depending on gender.
but 1 sounds like a good idea to start with, that way when its old enough to sex u can go from there.