In regards to interior Russian garrisons, this is something you want to do early to help with the happiness issues. Every citizen you have to have on entertainer duty is one less getting you cash, science, or civil improvement shield production. Russia has a huge allotment of "free" units before units start costing maintenance. Use that to your advantage.
I usually employ two strategies to help fill up those interior cities. The first is to slowly draft units from the fast growing cities of European Russia to help hold down their population a bit until I can get some mines in to up their production, etc. The second is I will station my reserve "fire brigade" units (usually cav) in cities that are connected by rail.
This is all early game stuff a Russian should be looking at while trying to get as many roads built as possible not only to improve their road net, but also cash/science as well and take advantage of that massive population. Russia also needs to forget early campaigns of aggression. You simply don't have the available units and what you have is not that hot. Don't neglect your navy either. You will need it in the Baltic and the Pacific. Build up your Black sea fleet if you intend to be aggressive with the Ottoman. At worst you can usually hold your own against the enemy fleet and in many cases, can use your fleet for needed shore bombardment to help the army assault coastal cities, because your artillery is not very good for a long time.