Gladius, you can't even fly a C172 or PA28 on a PPL without the required class rating (SEP). Anything more fancy than that requires an additional class or type rating, and there is exactly zero difference, in this respect, compared to CPL or ATPL.
In fact, even though you can add the Citation Mustang Type Rating to a PPL, you'll probably find that the exams for that type rating are at the ATPL level, or at least the CPL/IR level.
What you are probably confused with is the MEP class rating. This is a class rating that is easily within reach of a PPL, and covers all multi-engined PISTON aircraft up to 5700 kgs. But since the Mustang has turbine engines, it cannot be flown on a MEP class rating.