Um, ya, Norco is "Horse Town USA" and all, but you're talking about only really riding to a bunch of shops on one main drag (6th St). I know you can ride all over the city, but you're just talking primarily residential riding, a riverbed, an equestrian center and some limited back hills stuff. It's a very small equestrian residential city in the middle of a much larger, automobile-reliant, urban area. It's right off the 91 and 15 fwys, which are the hub for travel into the inland empire in SoCal. Norco is very horse friendly, but I can't imagine living anywhere in Southern California without a car. It will absolutely limit you as mass transit truly is not supported out here like it is back east.
I live in OC and drive out to Norco when I want to look for used tack or do a cheaper shavings run since prices are a bit better then they are here. But it's definitely not the kind of living I think the OP is looking for.