The only BIG issue with keeping ducklings and chicks together in the same brooder is the ducks grow SO much faster. Before a week is up, you'll see the duckies' webbed feet stomp all over little bitty chickies.
For that reason, I separated mine, but kept them in the same large brooder for two more weeks. I used a hardware cloth divider so they could all see each other, and interact, but not get harmed. After 2 weeks, the ducklings needed far more room, so I put them in a separate brooder of their own.
My bathroom is very full. 4 chicks in one brooder, 2 ducklings in another.