I think they would be best on layers pellets rather than just corn. I feed mine mainly layers and corn for a treat in the afternoon.
You won't want more than one drake for 3-4 ducks or they hassle them beyond belief. Should be ok until breeding season starts but keep an eye on them, duck sex is rather brutal at the best of times, let alone with too many drakes joining in.
As for eggs, I think they're quite likely to sneak away and make a nest. If not, they have quite a short laying period but you should get some

Dad reared some tiny Mallard ducklings one year, whose mum was killed on the road near them. One day they flew off around the next field and to the river but would come back for their afternoon tea for ages. Eventually they disappeared properly, though one or two still drop in occasionally
