Ugh. The Petco employee was clueless, but that should really come as no surprise. There are somewhere around 50,000 different kinds of mites. And they do tend to be rather specific in the type of animals they parasitize. A lizard mite is not going to be interested in a hermit crab, and vice versa. A reptile mite is generally not going to be interested in a mammal, etc. Many Petco stores just have so many constant problems with mites due to poor conditions that to them it probably just seems like one continuous infection. I can't even walk through their animal displays anymore, because I end up giving the section employee an earful when they're probably not entirely to blame - the company itself is.
Be careful with those mite sprays. They claim they are safe, but can still be harmful to your pet. Especially if it gets into their drinking water - and crested geckos are likely to be licking the condensation off the sides of the cage, obviously a place that you might spray. I would definitely not use it anywhere near your hermit crab.
Assuming they're not just springtails like Jess said, the best way to get rid of mites is to clean, clean, clean. Generally, eggs and larvae are not living on the animal itself, but in the substrate, so being fastidious about cleaning can break their life-cycle... but that means everything: Replacing substrate and thoroughly sanitizing any sticks, rocks, water bowls, etc. At least a couple times a week until they are gone. If you can see mites on your pets, rinsing them off with dechlorinated water can be helpful. Obviously, a hermit crab can be rather difficult to clean off.