Hi, iv recently started playing Descent 2nd edition, I'm now looking at adding an expansion, could i use heroes and monsters from any expansion with only the base game Shadow Rune campaign or do i have to use the entire elements of the expansion?

If you asking if you can play the monsters and heroes from the expansions in the base game campaign, then yes you can

Excellent, thanks mate, I wanted to add a bit more variety to the game without adding to much new rules