Dwarf lionfish will work in a 55. As far as tankmates, just make sure the lion can't eat it.... and they do have relatively large mouths for their size. Just think largemouth bass....
Since you posted this in the "reef" section, if this is going into a reef tank be aware that lions are poop machines, and more frequent water changes will be needed to keep nitrates down.
As far as rays... no, there are none that will fit into a 55. On top of them needing a very large footprint tank, they are considered difficult/expert care level creatures.
Wanna calculate the cost of that new reef tank?? Take your worst nightmare, double it, and you're halfway there.