My hatch of ducklings produced what I think are 4 drakes and 1 duck.

still not 100% sure, they seem to be taking a long time to develop their quack, at the minute it's more of a rasp but every now and then I hear 1 definite quack. I've leg ringed the suspected girl duck but she is still in with the rest at the moment as I'm not 100%.
So what do I feed to finish the drakes? Currently on growers and every now and then have rolled barley mixed in which they seem to like. Is this ok to finish on? Or should I up the barley? I don't really want to have to buy a finisher pellet as the feed bill is mounting up with chick crumb, grower, layers pellets etc but just wondered what everyone else finishes drakes on... and what age you could 100% certainly sex them? These are 8 weeks old now.