In a 75 I would recommend one of the smaller Zebrasomas (Yellow, Purple or Scopas). The Sailfin, while beautiful, grows large and fast - probably too large for your future 120. Any of these though should be the last fish you put in the tank - most are tollerant of existing fish, but once that territory is theirs they are not very welcoming to new arrivals.
The Kole or Cheveron are also excellent choices. If you go with a Kole, look for one that was harvested in Sri Lanka - they have spots on their body instead of stripes. Basically the same fish with a different paintjob. Both fish have an excellent temperment ans will be useful edition for algae control. They stay smaller as well so should fit well into your future 120.
I would stay away from the Acanthrus group (Powder Blue, Powder Brown, Gold Rim, Orange Shoulder, Sohol, Clown, Dussemeri). The powders have a terrible track record for survival and are prone to ich, the stress of being in a 75 gallon will not help that. The others, again while beautiful and good algae eaters, will get too large and require much greater swimming space than you future tank. This group also has the worst reputation for aggression, more so as the fish ages and grows in size.