The rules are as follows (taken from the Ops Manual, but in my own words I think):
On class 1 to 3 routes, you can fly class 1 to 3 planes.
On class 4 to 6 routes, you can fly class 4 to 6 planes.
On class 7 routes, you can only fly class 7 planes.
So you can fly turboprobs onl class 1 to 3 routes, and you can fly jets on class 4 to 6 routes, but you can only fly class 7 on class 7 routes.
The above is provided that your rank allows you to do both the flight and the plane.
If you use a different class plane from the flight, then you must submit the PIREP manually.
You cannot use VFA aircraft on Scheduled routes, however, some tours require you to use one.