I currently have a Tiny Terror with an Orange PPC112 cabinet. Can't complain! For my needs it sounds awesome! However Ive been wondering: if I get another PPC112 or just a PPC212 cabinet, will my overall sound change? Will it sound louder? Also can I buy for example a Marshall cab and use it along my Orange cab? Thanks!
If I were you I'd get a PPC 212 for a fuller sound.

You can use any guitar cab as long as the impedance is correct, brand doesn't matter. But if you're thinking of buying an MG cab, don't.
It'll be quite a bit fuller sounding, a little bit louder, a lot more thumpy bass on tap if you want it.

If you buy a different cab or one with different speakers, the tone difference will almost certainly be noticeable and possibly quite significant. Same noise, different flavour.