The males aren't going to show any bristles for a few more months. They are just babies that the LFS has. So...no way of sexing them right now. When you say you don't want a monster pleco down the road....if it is truly a bristlenose, the males get only a max of 5", females around 4", and most are probably a little smaller than that at adult size. If you're wondering if it is truly even a real bristlenose, take a look at the pictures on planet catfish or pleco fanatics. Could also go to aquabid to get a better idea of what a young bristlenose looks like. Most of the bristlenose that are posted there by breeders are usually between 1 and 1.5" when the breeder sells them.
Debbi