Of course Scats usually get upwards of around 10-12" theyre IMO the brackish worlds equivalent to a Pacu...they will eat literally anything and everything. Archers are awesome fish to my 100G brackish is beside my computer and if my Archers want fed they will simply spit threw the gap in the top of the aquarium and nail me with water. Colombians are also definatly mosters they get big...my LFS has a shoal of 10-14"ers. Monos are also a awesome fish to keep
I also think that the stocklist you mentioned would do very well in a 120 Heck my 100G has more than that(2 silver scats, 3 monos, 2 archers, 2 colombians, 2 puffers)