The one cavern level I have has all "tree"-types from all three cavern layers.
I also set cavern openess to 100 and passage density to 0, water min to 22 and water max to 33. This way I have huge open caverns which always will have some water. Really don't like the small-passage kind of caverns.
Creature-wise I found Crundles, Hungry Heads (lots of them), Giant Cave Swallows, Giant Toads, Cave Crocodiles, Troglodytes, Gorlaks, Blind Cave Ogres, Reachers, one Giant Cave Spiders (going by the list of dead/missing), one Cave Dragon and one Troll.
Downside of having one cavern level (on a 3x3 embark) seems to be that only one (or two?) group(s) of creatures ever spawns down there. My way of getting more interesting creatures than the constant crundles or hungry heads is to have two big strips of cage traps dividing the cavern level. At some point or another everything will run into them, getting caught and causing new creatures to spawn. (An army of egg-laying crundles is a good food supply though, I must say.)
Pretty sure everything can spawn, but most of the time till now I had a bunch of hungry heads multiplying in one spot, causing no new creature group to enter the cave; ever since I devised my trap-system I had a lot more variance in what ran around down there.
Weirdly the whole floor is slowly turning into sand, I'd guess it comes from more and more trees trying to grow but failing - either because too many already occupy that spot or at other spots because the ceiling is too low. I hope that doesn't cause no trees to regrow though...