When I put mine together finally, the big ducks were mean to the little ones for a few days and then the pecking order got sorted out and everyone calmed down. Make sure to keep a close eye on them when you do put them together, though, and make sure no one is getting seriously hurt. I would also put out multiple water and food dishes in case the older ducks try to keep them from eating or drinking. I hope everything will go smoothly for you!
Hopefully someone else will add on to this ;_; I'm very new to duck keeping and don't have much experience to pull from.
Thank you Tapioca, this is all good advice!