But all the fish in there need to be in groups. If you take care of them first, then the betta can go.
Harleys need at last 5/6
Black Skirts need 6
Clown loaches reach 14" long and need to be in groups
Yeah...the loaches will definitely outgrow that tank and really need something on the order of 120 gallons to thrive. The clown pleco and harlequin rasboras should make good tankmates, but black skirt tetras are known to be nippy so I wouldn't risk it, even if you did increase the number of them in the tank.