Its a good combination providing they are of compatible size,out of all the genus Mastacembelus fire eels are the most peaceful given their eventual size of 1 mtr!!!!
Retics on the other hand are relatively gentle feeders so any eel you put in with this species would be best if its smaller than the retic.....not so small that it may end up on the rays menu,fire eels can become incredibly greedy once established.
i have rays with a HUGE fire eel about 28-30" and it is a fine combo.. the problem is when the eel gets large, they can eat a whole bunch!!! it makes it hard to get food to the rays.... but my rays all eat pellets...