Just to confirm, the store had the rock in a tank, right?
Personally, I don't like the idea of putting in a cheap durable fish. The possibility of making them live in a poison environment seems a bit cruel to me.
If you have some fish food, you can dump some in if you don't want to do the cocktail shrimp route.
If the rock came from a tank at the store, I think it is possible you just aren't having all that much die off on the rock, so the cycle is "soft". If you're still sitting there after 3-4 weeks total, I think you're good. This was my experience on two tanks and they have all been in great shape