i would say they'd be ok in the 30 for now as long as the dempsey isn't too big. how big is it? from the photos, it seems he's no more than 6 inches, eh?
then again, an excuse to get another tank is always a very valid excuse! lol!
as for adding the dempsey to the 100 gal oscar tank, i've read about dempseys being fine tankmates for oscars. it all does depend on the individual personalities of the fish though, as well as the size of one fish relative to the other. i wouldn't put a 5 inch or smaller dempsey in with an oscar that is 10 inches or so - that's just asking for trouble, as the oscar may think 'mmmm..... dempsey snack!' or may harass the smaller fish.
on a semi-related side note: my oscar has grown up with a paratilapia polleni (black diamond cichlid). both are the same in personality (very mellow) and get along great. the oscar is a few inches bigger than the polleni, but the polleni is definitely the tank boss, lol.