I would advise against a Lawnmower Blenny in a 30. They get large, mean with age, and often starve in smaller or newer tanks. I had one starve in a 55 gallon. For a different Blenny, look into smaller members of the Ecsenius genus (i.e. Tailspot).
The tank is a little on the small side for a Dragonet, though if your tank is well established with plenty of live rock and a refugium, it can work. As far as your Gobies, I imagine if it were to get close to their burrow(s), they will chase it away.