Sometimes it is less of a hassle just keeping Discus by themselves, but I think cardinals (and a few other types of Tetras), Corys, BN plecos, and Rams do really well with Discus for the most part. These and Angelfish would be the only thing I would mix with Discus, but keep in mind Angelfish cannot always be trusted
As far as sevs go, I personally wouldn't mix them. I did hear though, that festivum are good tank mates with Discus, as someone mentioned, geos can be mixed too. If you do wish to try mixing them though, I would start with the smallest sev possible, and make sure you have another tank/divider ready.