Actually, Marx figured that out in the 1800's. If you have two classes of people, the more powerful class will take control of government and advance interests for themselves.
Not the same. Marx assumes you should have no rich people. Separating powers does not imply that. It just implies rich people should be isolated the other powers.
No, he doesn't assume you should have no rich people. He wants there to be only one class of people, regardless of total wealth. There are two classes in capitalism, owners and workers. In Marx, there is only worker-owners.