First of all, welcome to the forums. It's always nice to see new faces, even ones that look like :-)
The core sets are all designed off the same basic framework. Within one or two cards, they should all have pretty much the same frequency of monsters, even if you mix multiple core sets together*.
Something else you might want to try is the "Listening at the Door" Faster Play Rule, found on p. 6 of every new core rulesheet. (If your rules only have four pages, you have an older printing; you can download the most up-to-date rules
here, and look at the changes we made to the sets
here.) That rule is designed to get more Door cards into the game faster, so you have more options each turn, and to get more Treasures into the game as well. In our experience, it speeds up the game and makes it more fun, since you're seeing more cards each turn.
If this doesn't answer your questions, please write back and we'll take another shot at it.
* If you DO mix core sets frequently, you can also mix in
Munchkin 7 - Cheat With Both Hands (formerly
Munchkin Blender), which has plenty of monsters in its own right! That set should be out in a couple of months.