Annie,
I've got 6 chicks and 3 ducklings coming tomorrow and have been doing a LOT of reading on both.
I thought about keeping them together too, but there are two major drawbacks that I can see. 1) the ducks will grow much faster than the chicks and could end up stomping them. 2) ducks should not eat medicated chick starter. In fact, ducks really should have special duck or gamebird starter, as they require a higher protein level and niacin, which the chick starter doesn't have. You could probably get away with using unmedicated chick starter in a pinch, but absolutely don't give them the medicated. Apparently they aren't as prone to coccidiosis (sp?) as chicks are and they eat a ton more, so consequently could overdose on the meds.
Hope this helps! I'm very psyched about my duckies - even though I now know I'm not getting exactly what I ordered. Oh well! C'est la vie!