I tried the same idea when I setup my first tank many years ago. You'll benefit more with using less dense rock. The little holes and crevices allow for better microfauna numbers and hiding places for fish. Provided the rock isn't something like limestone (like you mentioned) you may be able use a few pieces as base rock.

After battling different pests (ie byropsis, derbesia, acro eating flatworms, etc) I'll personally never use "live" rock ever again. Just buy a bunch of dry live rock and seed it with a piece of established live rock from a trusted source. You're tank will be ready to go in no time.