I suggest you post the same question on a specialist livebearer forum, rather than getting the same vague guesses you usually get here. (Seriously? ameca splendens?).
As for Gambusia affinis, the female lacks the usual "teardrop" under her eye, and the male just looks different (certainly with those tiger stripes).
Good on you for trying to get an ID. I really hate when stores sell fish with no name