What is the best substrate for Russian Tortoises? I know that a lot of people use cypress mulch, but I've heard that it is good for larger species, not the Russian Tortoise.
...and may we know appx. where in the world you are?
In my opinion, you can use whatever type of substrate works for you and your tortoise. There are some that you should stay away from, like walnut shells, alfalfa pellets, sand, or any of the paper products. But coco coir, dirt, cypress mulch ... any of those are ok to use.
You can choose to use more than one substrate in an inside enclosure. For example, you might have cypress mulch on the warmer side, including the water dish, as it is less messy. On the other side (separated by larger rocks, or a log, or bricks, etc.), you can have coir mixed with organic topsoil.
I have used cypress, peat moss, sphagnum moss, and combinations of peat with cypress on top, but I just recently started using organic garden soil and I like it the best. You can pack it down so that it wont be too messy, but they can still dig into it when they want.