I was making a custom monster to go with a mini I have when I realized that I'm not sure how the "hybrid" classes should interact with the rules for advancing monsters with class levels. The "chassis" I decided to build on was an Aboleth with the Young template (since the mini is Large rather than Huge)
The Aboleth has pretty good Charisma, and it seemed fun for this monster to be able to act in a support role before it moves into combat directly. I figured that a Bard might be fun. The Aboleth (perhaps somewhat oddly) is classified as a "Combat Role" monster, so Bard would be a non-key class. Then I thought about Skald. It is mostly like Bard and gets roughly similar combat bonuses (+4 Str/Con vs +2 or +3 to attacks and damage at the levels I'm looking at) along with a few Rage Powers. Are those enough to turn a non-key class Bard into a key class Barbarian though?
I'm inclined to say yes since I'd rather be a little conservative with CR, but I wonder if anybody has seen guidelines on hybrid classes for monsters somewhere.