Cheepcheep,
I did something similar just the other day. Ordinarily I would suggest the same thing as Miss Lydia...That advice is solid and is what I do with my chickens. However, your initial 2 birds shouldn't have a problem bringing in another 10 birds in a few weeks. Here's what happened to me:
I had a single pekin hen (her other 2 flockmates were the victims of a *now dead* coon) who has been alone the last 4 months. I picked up a pair of ducklings at the local feed store about 5-6 weeks ago, followed by another pair 2 weeks later.
I introduced the first pair at a month of age to their new digs (with the adult pekin) two weeks ago -- followed by the other 2 ducklings this last weekend. Yes, there was a bit a squaking and nudging about as they all "learned their place". But at the end of the day they were all swimming nicely in their pen and snuggled down together that night.
Ducks are MUCH less vicious than chickens, it seems.
Like I said, you shouldn't have a problem -- just keep a watchful eye out and be prepared to play referee if necessary.
-Randy