Balancing Classes

I have all 4 books that contain character classes. Are they mixable, or of I'm running Heroic I only use the classes from Heroic, or is there some kind of conversion tool I can use to scale between the books.

They're all built on the same systems and are all mixable however you like.

In fact, there's a guide in Heroic on different ways of mixing and matching the heroic and core classes to build different kinds of campaign.

Incidentally, the rules for building classes are in the Player's Companion (which really ought to be called the Rules Companion, since those particular rules are more for GMs than players.)