I'm after some help understanding what I can replace my current rear mech with on my mountain bike. I have an 8 speed casette/shifters etc. Can I use any rear mech regardless of the number of gears I have? The number of gear shifts depends on the shifters and the rear mech doesn't know how many shifting positions the shifters have so I guess I can use any. Help appreciated.

yes, just need to make sure that you keep the length the same

Length? In terms of short cage or long cage?