For the most part, you have to have exactly the same number of leagues and countries that currently exist available in the game. Crucially, the leagues have to have the same number of teams they currently do.
If I were going to tackle the process you're going for, I'd probably work it this way:
Country changes
- rename Russia to "USSR"
- give all players from Soviet United countries "Russian" as a second nationality.
- cherry-pick the best for the national team, and change their "caps" from their current country to Russia.
League changes
- use the "swap teams" function to move the key teams into the Russia league structure.
You really want to go ask this question in the
Editors Hideaway forum, though - there's a bit less traffic there, but people with a lot more experiencing editing can probably give you a better answer to your question!