Is this correct?

If a turbocharger is too small for an engine, it will restrict intake
airflow making it perform worse than having the engine naturally
aspirated.

If the turbine side is also too small then then the exhaust will act
as a restriction at increased boost levels.
And one way to solve this problem is by using an external wastegate,
right?

I know I'm possibly complicating things.

Using a turbo matched for the engine would simplify things
BUT I'm what I'm trying to do is come up with a compound twin turbo
system (different sized turbos) which would significantly reduce turbo
lag without limiting boost.