Corys are not appropriate for tanks under 10g. They are schooling fish, they need to be in a group. And they need room to move, floor space if you will.
your cory needs more room as they can get up to 2 1/2 " long,and he will be very lonely without otheres of its species,its the same with oto's.either you can get at least a ten gallon or get rid of everything exept the betta.