Welcome aboard!
Beach pebbles look pretty, but generally offer little in the way of habitat for nitrifying bacteria, as they're not porous like live-rock. There's also a possibility that they've got some undesirable chemistry on them, if they're from someplace like an inner harbor - or even that they might be a type of rock that can alter the chemistry in your aquarium.
That having been said, most are probably pretty harmless, and can make for a pleasing accent - and if you're keeping local coldwater creatures, they're probably very appropriate decor.
~Bruce
