Just buy one male and 2 females for a really good chance of gettin a pair, i just bought 1 of each and got a pair. so look for one with no red belly and 2 with and you should be good, oh the fry are pretty slow growing it has been about a month and my fry are only just hitting the 1/4 to half inch mark
yes thats the best way as if you get 6 the strongest pair will form but if you get a male and a female they could be crappy ones in the genes. so go with the 6