Yes, Cories need to be in groups of at least 3 because they`re shoaling fish.
If you`re looking for tank buddies, you can put in a few Ghost Shrimp, a snail, or a pair of african dwarf frogs. Those are suitable tanksmates for 6.6 gallons.
Puck (Yellow koi HM king) / Scar (White HM) / Salt 'n Pepper (Silver HMPK) / 'That fish' (Copper butterfly HM) / No name (White DT) / No name 2 (Turquoise/white marble HM) / No name 3 (Lilac HM)
Bo Peep (Cambodian VT) / Goldilocks (Yellow VT) / Joie de Vivre (Multicolour VT) / Lil' Peep (Blue VT) / No name (Dalmatian VT) / No name 2