As long as they are the same age, you should have no problem raising the two together. I've done it many times. You should really get at least 2 ducklings though to keep each other company. My ducks don't really hang with the chickens. If they are raised together as babies they'll stick together outdoors for a little bit but then they go their separate ways, so a lone duck will be just that. I'm sure some people have one duck that stays with their chickens but still it's better to get two.
As for the medicated feed. If the chicks are healthy. its not really necessary. You can have a finite amount available to them in the brooder. Like a small cup so they can all get some in them but there isn't a real need. If a duckling eats a bit it'll be fine. They not supposed to eat it all day everyday though, so people say anyway.