yep, mandarins are great mixed collection birds and do fairly well in larger groups, they are also cavity nesters.
Shelducks on the other hand aside from a couple species of them are very aggresive, and must almost always be paired and in individual "shelduck" pens with just that pair. They for the most part are burrow nesters, with 1-2 species just being ground nesters.
What I always did for mine was dig a hole near the ponds and put a 5 gallon bucket in it on it's side. Drill many holes in the bottom side for drainage. Then bury all but the mouth of the bucket for them, they really loved this style nest.
The only ones I have ever mixed in collections are the common or european shelducks. They arent nearly as agressive as say ruddies or paradise and do good in large community pens. My Radjah also did well like this, but in pens away from the europeans.
They are also more of an on land/grazer species and dont require as much water as other dabbler species in the pen.
they come in several species, european, ruddy, radjah, cape, australian, paradise, all very large and very beautiful, but for the most part, bullies.