Researching adding some ducklings to the flock. Advice???

[Florafawn]

We have:

1 welsummer
1 sexlink
2 cuckoo maran
2 mottled java

All are 6 week pullets. We have been wanting to add 1-2 ducklings to the mix (separate coops for now while little). Eventually we'd like to house them together. They'd have roughly 1.5 acres to free range during the day. Cooped at night.

And suggestions on breeds?
Any advice? Anything you wish someone would have told you?

Thanks!