I agree that Cories are great tankmates.
In a 125 gal tank, you can add almost anything that is compatible with discus - examples: Cardinal Tetras, a school of 30 or more; Rummy Nose Tetras; Harlequin or Copper Rasboras; 6-10 Rams - Bolivians, or German Blues. Pretty well any good sized Tetras that you like, Lemons, Glo-Lites, Serpaes, Head & Tail Lites, whatever.
In a tank that size, some would even keep 8 -10 Angels with them. Depends on what suits you. Let me know what you're thinking, and I'll try to advise whether they're compatible and can tolerate the higher discus temp.