Apples & Oranges
If my memory serves me correctly the kind of operation you are doing is what determines whether you need a PPL, CPL or ATPL (you can add employer and insurance requirements to that also but it's not regulatory by the FAA). Pleasure = PPL, Commercial Part 91= CPL, Part 135 and 121= ATPL.
Type Ratings are required for all aircraft over 12,500# and TurboJets. Thus you can have a PPL with a Boeing 757 Type rating. Or you may have an ATPL with no Type Ratings and fly any non-turbojet aircraft under 12,500# for which you have a license for (as in Aircraft SEL, MEL, Rotorcraft-Helicopter, etc.).
The Type Certificate for the specific aircraft determines how many crew members are required. B-757 = Capt. & F.O., Bell 47G-2 = Single pilot, DC-7 Capt., F.O. and Flight Engineer. Note here: Only the Captain needs to be Type Rated. The F.O only needs to have a license in the category (Airplane - MEL for a say a B-757). In this case the FO could be FO in any multi-engine requiring 2 pilots but cannot be Captain in any over 12,500 or TurboJet aircraft until he is Type Rated.