It is a shame you did not ask this question before you got your lionfish. If your lionfish is a dwarf (we hope) then your tank is a squeeze for him when he is full grown ( 5 or 6 inches). He will eat fish smaller than himself, and you don't really have the room to get another fish that will match the lion in size. Also, unfortunately, lionfish are fairly high polluters.
I'm thinking you may have backed yourself into a corner with your choice of that first fish.