they're sexable, sometimes, at that size. I breed albinos and can usually pick out a few males at around 1.25" or so. It's much easier to find a male than a female because not all fish without bristles are females. Sometimes, they're just slow males. You can also sometimes see differences by the shape of the body and head. It's hard to describe, but you can definitely see two different shapes if you have a bunch of young fish together. Some heads are round and some are more angular, when viewed from above. The angular are generally all males. But, yeah, it's a crapshoot at the store when they're small. If you know the fish are from the same batch, pick the largest and smallest and you'll get a pair 95% of the time. They should definitely be sexable at 2", given good food, space, and lack of competition. A lone male should get bristles by that size, which will take another 2 months or so from 1.5". Feed it well and it should be 3" in 3-4 months from then if it's male. 6+ months for a female.