Actually, RDM/NIN makes a pretty good combo if you want to be mainly a malee RDM on support healing. Especially when you get Duel Weild, you can equit two Ctr Swords at lvl30 or two Holy Swords at lvl36.

NIN/SMN I would say is the bad combo, if you want to have some fun with your character then I would suggest maybe DRK/RDM as thats a rare, but powerful combo(also highly loved once you get to PT with one)