[Van down by the river]

Basically, if it's too small to attach to they will eat it.

They are extremely fast swimmers with a strong tail equipped for bursts of speed. If you choose to go ahead anyways and put them in a tank on their own they will spend much of their time stuck to the tank wall like a pleco.

As to your "spotted grouper" this could be any one of many species of grouper.
I suggest you try to get the Latin name so you can ensure your info is accurate. Also when people give you advice, you can be more sure that you are all talking about the same fish. Generally I'd consider a 100gal as minimum for any grouper and many species require a larger aquarium.

They have a similar rule to Remoras;
If it (their tankmates) could fit in their mouth, sooner or later it will!

On the plus side most species are very hardy.
If you let us know what species exactly, I'm sure you will get more detailed information.