Everything is compatible.
Every expansion from every edition is compatible with everything else.
You can play with any combination of expansions.
If you somehow have 1st or 2nd edition then there might be a minor rule change or two.
3rd to 4th edition had the 'biggest' change: The symbols are different, and there are now "player boards" and "life counter tokens/bullets". None of which actually affects the game.
High noon and Fistful gives you a separate deck of cards.
These are the only two that have special rules with how they work together:
You pick a dozen or so cards total, and then choose either finisher for the end.
Wild West Show has a similar-ish "events/effects" deck, but it runs on a different 'trigger'.
Dodge city has editions too. Older ones are not fully compatible as you are missing one player board.
Not having a player board/life-tokens does not make it "not fully compatible". The boards are not required to play the game, and are purely cosmetic.