I find them about as often as the other biomes just generating new worlds over and over. They're even easier to find in small biomes worlds, although there's usually a few trees and that's it since they're so small in those worlds. I've found quite a few in normal worlds as well.
The Meaning of Life, the Universe, and Everything.
No its not compatible because this is a ModLoader mod and ExtraBiomes XL is a forge mod.
I'm making a forge version to hand of the the mod author shortly though, so hopefully compatibility will be made.