Hey guys. I just bought Labirynth of Ruin. Can I use the expansion classes and rules in the base game's campaign?

Or am I restricted to the core classes and monsters?

Use everything you have!
It's really ok, since the OL gets monsters and new class cards he can balance out the items and classes the heroes get. Go nuts, have fun!

Add every component from the expansion to their respective supply, as instructed by the rulebook. This also includes heroes, classes, monsters, ...