Is it okay to have these fish together in a twelve gallon tank?

The "rule" 1 inch of fish per gallon of water is not "TOTAL CRAP" but is a perfectly good although archaic means of working out how much live stock a particular size tank will hold without too much detriment to that livestocks health, so therefore I would suggest that the stock of fish in this tank is the absolute maximum. Don't forget everything grows and in a few months all your livestock will be bigger than they are now.