Discus are happier in groups of 5 or more, yes the will sort out who's the Boss.
Loved my Discus when I had them, have now changed to Africans, alot more active than the Discus, I found Discus are more of a bobbing fish if you no what I mean.
Yes, they can be kept in a Community Tank, as long as the Tank Mares are abit more docile, Discus do seem to get fright's easily, so slow moving tank mates are better for them.