HI:
2 or 3 dwarfs, or 1 dwarf and 1 mid-bodied lion (antennata, mombassa, radiata). The problem will not be directly space related, it will be waste related initally, then when the fish reach max size you'll experience space issues.
I will say that having a tank of lions while sounding cool is actuallyvery slow paced (I know). you'll end up getting a few swimming fish just to see movement, as all the lions are sedentary predators. So I will almost guarentee that this 55 will have more than just lionfish.
So my suggestions would be for all dwarf lion tank
1)-you can keep a Fu D.biocellatus, a dwarf fuzzy (D. brachypterus, and a dwarf zebra (D. zebra).
I will say that if you want these type of fish in that small of water volume,you should concern yourself w/ water quality and buffering issues.Ensure you have lots of live rock, a DSB, or a wetdry and the best skimmer you can afford
Also be constistant in your water changes.
Lastly, lionfish have to be weened over to dead prepared foods
frank