Yes the vetrals. With males they are typically more jagged looking almost forked, females are smooth looking.
I am not to sure if they choose pairs. I know the ones I have had they seem to choose who they like, at the moment I have only 3 but 2 of them are always together, but they dont want nothing to do with the 3rd one.
Yea thats the sad thing that i noticed with oscars, chiclids and other tetras.