A 50 gallon isn't very big so you might be limited on what you could add. With that said, you could probably do another dwarf species of lionfish or possibly even a hamlet would be cool. Stay away from the more slender body types and while usually fish of equal size is a good rule of thumb, they can have a healthy appetite at times and their eyes are bigger than their stomach.
I had about a 2-2 and 1/2 in Radiata one time and put it in a tank with a clownfish of equal size and I woke up the next morning with a dead lionfish and half of the clownfish out of his mouth. You say you have no Coral another thing that you could do and I currently have a 20 gallon set up this way is you could do a macro algae display tank and that would help keep the nutrients in check with a heavier bioload. As a temporary home until it gets bigger, I have a medium body lion that's about 3 and 1/2 in a 20 gallon long with the macro display and I have to add nutrients to the tank.
