You can introduce adults to your pond... the issue is cost. Once you realize the cost, you will want to buy day old baby ducklings and goslings as it's a better value... although it takes more of your time to handle and grow.
I don't see an immediate need for an island. The water itself gives...
Two acre pond? My god, you could get 200 ducks easily. With that much room, I'd get geese as well as they're excellent 1-2 punches as far as weeding and eating insects.
My personal favorite ducks:
Silver Appleyards
Saxony
Welsh Harlequin
Geese:
Pilgrim
Embden
American Buff
Chinese
It's...