They're going to need to fatten up, and grow just a bit, before it can be said for sure. However, they all look to be juvenile males. Fish #2 may be female, but the best sexing pics need to be almost perfectly straight down from above on a solid surface.
Larry
Impossible only means that somebody hasn't done it correctly yet.
Interests: Fish of all types but mostly Loricariidae, Callichthyidae and Auchenipteridae. My awesome dogs and walking through nature and taking pictures.
That will be very difficult. Specially because the fish will come from the wholesaler and will be very stressed and thin due to transportation.
Pick the fish with no odonthodes on the tale and pectorialfinray (feel the tale with your fingers), very short (or no) odonthodes at the jawline and a small and narrow headshape.
That's the best you can do I'm afraid... Good luck!!