Yes Nathan, you can put almost any scope on any mount. Theoretically
They aren't restricted to Brand or style.
But they are by size.
You wouldn't put a 12" Reflector or a 14" SCT on an EQ5 as an example.
It is always a good rule to overdo the Mount.
Mounts need to be overkill if you intend to do Astrophotography.
2nd hand EQ6's are ok if you can find one at a good price. If it is close to new price, just buy new and get a warranty.
As for matching mount to scope, you can put an Orion scope on a skywatcher mount, or a GS scope on a Vixen mount etc. They are almost all interchangeable.