What other breeds of tortoise can I keep with my Russian?

No, bad idea!

Never mix species or you could end up with either one getting extremely sick from pathogens that the other has natural resistance to and can carry without symptoms.

You could only keep other Russian tortoises with your existing tort., and even then getting just one more is not a great idea (if yours is definitely a male?) Basically, if you are keeping more than one tortoise, and one of them is a male, then you should then have a minimum of 2 females, so that neither female gets too harrassed.

Ideally any new additions should be of similar size to your tortoise, and you MUST quarantine them for a minimum of 6 months before housing them all together.

Hope this helps?

Different species should not be kept together, they can bring in disease that while not harmful to say a Hermann might kill a russian. Also most tortoise species do fine all by themselves, they often view other tortoises as competition for food, basking spots, and such. If you want to get another tortoise and keep them together you must get one of the same species, if you want to keep them separate then you can get a different species. Remember the minimum size for a tortoise enclosure is 1 square foot for every square Inch of tortoise doubled for more than one, and tripled for three.