I can't give you a specific species on the mites, but if you desire such, you can usually take a sample to your university's entemology department.

I can tell you that there are thousands of species of mites and other invertebrates that fit your description. They are very common in the type of setup that you describe. I have them in a good half dozen of my enclosures. For the most part, they are a good thing. They help convert detritus into usuable components for your plants (or just parts no longer detrimental to your herps). I have not heard of any species that resemble these little bugs that are parasitic to herps. While they are commonly present on inverts (commonly millipedes), I do not know if they are harmfull to them or not. I don't think they are.

I hope this helps,

Ian