Severums are pretty mild tempered. If they start fighting in your future 120 just put in some vision barriers and redecorate. I would get about 3 more sevs to add once you have the 40g cycled. Once they start approaching 4-5 inches you could move them to the 120g. As tankmates for your sevs in the 120g. I would go with a school of 5 silver dollars and maybe 4-5 clown loaches.