Blood worms won't do anything at all for a ray. I would just 86 them. Krill is great for stingrays. That's my ray's primary food of choice.
I feed my rays- krill, night crawlers, silversides, tilapia.. And my pike eats massivore, so sometimes, they get that as well. I have even fed mine clams, mussels- both out of shell, and even octopus. They eat anything.
I wouldn't keep it at 84 only because at higher rates fish metabolize faster, so you would find that you're feeding a lot. I keep mine at 76, and they are more active than ever. When I first had them in QT however, I did have the water in the 80's. But I gradually decreased it over time.
Fx5's use 50 watts of electricity, so have fun with that. lol. I have Eheims on my ray tank, and the 2 combined don't even use that much. Also, with an fx5 you will need to add more media, and yes, you could put it on another tank, but I would just put the tank together, and throw in some seachem stability, and let it do what it needs to do.
Sponge filters are great for Bio. I always keep sponge filters in my tanks (even if not needed) so that if I ever need to start another tank, I have something with bio on it.