Going by the 1 inch rule, your nearly full but you could add 2 of either guppies or platies - make sure they are the same sex or you'll be inundated with fry . Avoid mollies because they can grow too big - about 4 inches. HTH
i reckon you can put some sword tails in because they make a nice feature as well as fancy guppys. Add some java fern and other plants so incase you have babys they'll be fine.