Weird, I know I answered this but I guess it disappeared. Or I was so keen on getting out of the office I forgot to hit submit!
Anyway, yes a 120 would have enough real estate for a Scooter and a Red Mandarin to coexist. It would also be enough room for a mated pair of Mandarins. For safety, though, I would not keep a Scooter AND a mated pair of Mandarins. I think that might be too many copepod-eating mouths to feed. Granted a scooter blenny is a good candidate for converting to frozen, but I don't know if it's worth the risk factor involved.