At this size it's hard to be totally sure. I'd bet male, but could be wrong. Your best bet to get stable pair is to chat to the guy at the fish store and explain you have one fish already and want a bonded pair and take 5 fish home with you tomorrow. Give them time to pair up naturally and then remove all the spare fish and agree that you'll take them back to the store (larger, better fed and probably in better condition!) in return for store credit against future fish purchases. By letting them pair naturally you will get a much stronger pair and then you will stand a much better chance of them breeding in your tank. You'll also get to see a load of natural behaviour as they fight and spar to select mates.