IME, the only successful mixing of blennies was a combtooth blenny and a fanged blenny in a 40. The two had no interest in one another.
I tried mixing two combtooth blennies, and I wished I had not. Also, lawnmower blennies are extremely aggressive, and I would never try mixing another combtooth blenny with one of these. I have one in a 150g and am certain he would finish off any blenny I added.