I think it would be an absolute sin to have rescued them, then to kill them for the freezer, so I hope they really do have a good home now. I know you and Lynda are softies really
Unfortunately I have a dog that chases and catches hens and ducks so if I had any they would have to be penned, but if you only have one drake plus your oldies you shouldn't have any problms.
You have added another string to your smallholder bow as duck eggs are sought after.
The best way to be sure of their sex is to spend time with them adn listen to their voices. Ducks quack drakes just squeak