In my opinion, a school of Tinfoils and/or Balas will outgrow a 120G. I regards to Serpaes, I would be worried that they would end up being lunch.
I have a school of Arulius Barbs (Puntius arulius) that supposdly grow to 4 or 5" (they are only 2" now). To say that they are active would be an understatement (these fish are hyperactive). They school nicely and have awesome colors; the pix I have found online don't do them justice...mine have a deep burgandy tail and light yellow head/torso. I believe adult males grow long streamers off of their dorsal fin. I really like these little guys. I'm currently keeping them with a JD, GT, Oscar, Wide Bar Silver Dollars, Clown Loaches and some Yellowtail Rasboras.
Another option which grows a little larger than the Arulius barb is their cousin, the Filament Barb (Puntius filamentosus). Roseline Sharks (Puntius denisonii) are another attractive option that should work nicely.
My LFS has some fish that are very simular to the Filament Barb (Puntius filamentosus), but have a black spot over their eye as well. Extremely beautiful fish, but he has them labeled as Madagascar Barb and wants $35 a piece for them....