Yup... you'll never see the nassarius until you put food in the tank. Then they'll pop up out of the sand bed. The others will be on the glass and rock, and when they're on the rock they'll just blend in. Many folks say "ewwww... I hate snails, I don't want them in my tank", but they really are a necessary part of keeping things tidy. Without them, you will pretty much be guaranteed an algae problem.
There's no right or wrong regarding whether or not to put them in first or later. There's just no reason though to put them in later. In my opinion, the sooner the better. If you go too long without them, you'll be playing catch up with the algae.
If you're adding live rock in stages, just remember that unless you add FULLY CURED live rock (or completely dried out base rock), you'll most likely see a mini cycle with each new addition of rock, fueled from the die off from that rock.