seems to be confusing as juvies, which is all I have ever seen offered for sale. couple things to look for though.
males usually have a wrinkled pattern behind their eyes. they look like fine blue lines and do resemble wrinkles. they also develop a stripe along their belly although this really comes into play as they mature.
they are also less "round" and more torpedo shaped as they mature.
females-more round and no wrinkles
from all i've read and experienced the best and probably only practical thing to do is get a group and as they mature select the ones you want to keep and sell the rest.
with my tank I have at least 4 mature males. they fight, they swim away, they hide, no real issues because they have a mess of plants and a big enough area nothing gets too serious.
In a small tank I could see real problems occurring. the fighting among the males is not gentle play--they can be pretty viscous.
just my OPINION based on EXPERIENCE. not FACT
when i rescape my tank I will increase their number to at least 20 if not 25. we will see....
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry