if you found a home for the Ray, you could certainly add another bala in my opinion to a 180. A 180 is enough of room for balas, even if they 14 inches long at full size (IMO).
Heck I jammed a 12incher into my 90 for a few years (his friend the other big shark jumped out..not the greatest tank, but it can be done.
However, there is a big problem here that has not yet been mentioned (I think unless I am blind):
If you have a 180 with a couple of big balas and a big fire eel fine.
No problem there in my opinion.
BUT this is the problem with the ray:
there are VERY SENSITIVE to water quality.
and you will have reasonably high nitrates no matter what with 2 or more great big balas and a great big fire eel, especially feeding them the cut and live foods that you are.
That ray have one heck of a time surviving in those conditions.
he is okay now. but he probably won't survive in the long run.
They are not hardy fish.
They are very sensitive.
And even in a big tank, you will have high nitrates with all those giant fish. guranteed.
The Ray just won't make it.