They tend to be pretty easy-going, concentrating on their own business---if they're the mostly algae suckers, which I think are the comb-tooth blennies.
There are, however, the fanged blennies, which I think are a bit more bitey.
Main thing with a blenny is don't put competitors in with him, and don't put in things that have the same shape and style---he'll notice that and may take exception to it.
The lawnmower blenny is good-dispositioned. His only fault is a little disposition to nip clam mantles and some sps polyps, so I understand, but on the latter question I can say I had one with lps corals and had no problems.