First day ducklings arrive let them rest and recoup from their trip.[That will also give you time to watch for any problems] Then let them meet your only duck and see how it goes. Hopefully they will hit it off great but since your duckling you have now will be older and stronger you'll need to watch in case the older starts picking on them or stepping on them. If this happens then you can make a brooder so that they can still see each other but older can't get after the younger ones. Hopefully it will just go smoothly they will all fall in love with each other. Great news about your son too.