Tanning isn't really hard, it's time consuming and kind of a pain. I suspect most taxidermists send out their mounts to be tanned because it's more cost effective to have someone set up to do it.

It consists of soaking in an acid solution for a while, then lowering the ph to a neutral, then stretching and scraping the skin and washing it. It's been a long time since I did any tanning. One of the taxidermy supply companies - Wasco actually sells tanning kits if you want to try it for your self - they also sell small animal mount kits (for things like squirrel).

For small things like rats and mice, most taxidermists have them freeze dried and don't even both mounting them. They can be tanned, but don't need to be even squirrels don't need to be tanned to be mounted.

I do know one professional tannery that does rodents, but only for fun.

Cheers!