You can get by with snorkel fins in the pool and for certification as you have, but the shop is correct in saying they can get you in trouble. Sooner or later you are bound to be in a situation where you need more powerful fins. Even if not you may find diving more enjoyable with less effort to get around. .
You can get away with pretty much any scuba fin to snorkel, even though they will probably be bigger than you might need or prefer. Fins that are at the extremes of positive or negative can be annoying when snorkeling, if your fins are always breaking the surface or dragging your feet down. Sort of depends how floaty (or not) your legs are. Buoyancy of fins can be something to consider for scuba as well, and you may want something different for that, so it can be a compromise.
What to get depends where you dive and maybe what your priority of snorkeling vs diving is. Going barefoot and not having to mess with booties can be nice for snorkeling, and is ok if you only dive off boats or benign docks and beaches. But open heel fins that require some sort of booty are more versatile - you're better off with real booties if you need to walk around much before actually getting in the water, like some shore diving. Using something with socks or really thin booties is a sort of compromise. They'll protect your feet a little, but will get ripped up quickly walking around much on any unfriendly surfaces. And it's hard to find fins in a given size that will fit properly and work both ways.
I agree the Scubapro Go fins are worth looking into.
