I also vote for female, on colour and finage. Yes it is possible to pair them later, I paired mine at 6 inch using 2 females for the male to spread any aggression......and of course he wanted to breed with both. I believe it is more the female choosing to accept the male, I had one male that none of my females would accept to breed with and my current guy that they all want to.
Males, even at the size you say have more metallic sheen as Tankboy mentioned and this should be noticable in quality LFS tanks.
good luck with your breeding project
cheers
Mudjimba