As far as I know, Mantis shrimp aren't found in California except in tropical fish stores.
GPO's are much more common in Washington, BC and Alaska but we occasionally see one in Northern California. I've seen two over the past several months. I know many divers who have been diving for years and never seen one.
I haven't tried using a snoot yet, but the challenge would be to maintain your aim of the snoot at your subject while maintaining your body position, camera position and setting up your shot. Aiming a strobe without a snoot can be challenging.
We see many Wolf Eels in Monterey. Lately, we've spotted one on most dives we've done in Monterey Bay and Carmel. You have to inspect every large crevice with your divelight to spot them.