I agree with you cacichlids body and fin shape are not the most accurate ways of determining sex of most central American cichlid, especially common tank bred varieties. Venting is the easiest way to be certain on sexually mature specimens. I can tell you right now though that if this is not a female Texas at 3" - 4" it is a very subdominant male that should not be bred.
I do think its a female and by the pictures it is obviously stressed (tattered fins) and it looks to me like a female just entering sexual maturity.