You don't need any capital to start a coordinator, liquidity comes from the users. 50 inputs is an estimated lower bound for a WabiSabi coinjoin to provide a consistent amount of matching outputs.
That is 50 inputs of reasonably large vales - obviously I can't just load 50 0.01
BTC inputs and expect user with 1
BTC to get their input properly anonymized.
You are correct, a larger value cannot be hidden inside a smaller value no matter what coinjoin protocol you use because it would create a negative value for the change. It is literally mathematically impossible.