None of the tanks described, in my opinion, are big enough for any of the fish you mention. A Red Terror needs a minimum of 75 gallons for a single adult fish, you might get lucky and successfully keep a m/f pair in a 75 but you'd probably have to use a divider in the end. The same is true for large flowerhorns as well.
Discus are another story, you might be able to keep a pair in one of your tanks (40/55/54 corner) with NO other fish. Be prepared to maintain uber pristine water quality though. High quality aquarium filter, RO/DI water filter, etc, etc...
As for stingrays...
Rays live on a horizontal plane. To limit their range of movement by putting them in a tank with such a small surface area (40/55/54 corner) is not only cruel but you could kill them. Do you currently own the 300 gallon you mentioned?
Rays, Discus and yes even flowerhorn = Bad "Beginner" fish.
All three require experienced, dedicated, care. All three can be sensitive to water parameters and less than pristine water quality in general. All three require basic knowledge of a proper diet plan. All three can develop ailments that are dificult to spot and difficult (sometimes fatal) to treat.
If you are determined to own one of the fish mentioned in this thread. Find a mentor, listen to what that mentor tells you, study on your own. Dedicate yourself to sifting through all the crap info for the real deal. Theres a whole lotta google out there, use it.
p.s. Discus can give rays terrible diseases that are extremely difficult to treat.