I've started keeping some keyhole cichlids, and they seem to be breaking the mold when it comes to conspecific aggression in dwarfs, but I don't know if they would be equally peaceful with cories. You would also only want to keep only one as a centerpiece as they get 3-4in. Very slow deliberate swimmers though.
12g is perfect for most dwarfs, but the corys will need to be traded in for most species. Trust me when I tell you, they will be far more rewarding. Just don't start with blue rams. they're reputation as a good "beginner dwarf" is based on their price and availability, not hardieness.
Not looking to breed. A single apisto male and 5 tetras all of the same species would make a stunning display IMO.