I've never had an undulated trigger, but from my understanding they are one of the most aggressive triggers only out done by queens and titans. From everything I've read, they will eventually kill any other tankmate, it will only be a matter of time. Best kept to a species only tank of at least 40g (US).
They won't kill the LR, they may chew on it a bit and if there are any living things on the rock like sponges, tunicates, corals, etc...they may end up being tasted (which may indeed kill the organism but it isn't intentional) but that's about it.
As far as a clean up crew, well I have two triggers with the picasso being the far more aggressive of them, and he did try to kill the snails but (1) I have two very large turbos and a large whelk and (2) these snails have learned to only come out at night! During daylight hours, they are solidly attached to something and completely withdrawn into their shells. I also have a very large hermit crab. The one I have is about the size of a golfball and has shown the ability to defend himself from the trigger. How long he will be able to do so is anybody's guess, but I think he has a good chance especially since the trigger now accepts him.
Again, I have no personal experience with the undulated but I have researched them quite a bit as at one point I had thought of getting one.
HTH
