I believe that the 1989 Peugeot model range used freewheels on all the 12 speed models. Cassettes were not employed until you got into 14 speeds with the Triathlon model.

Certainly a 7 speed freewheel will thread on in place of a six speed. Plus, it allows you to upgrade to HyperGlide's profiled and ramped teeth for improved shifting. However, a 7 speed freewheel is marginally wider. There will be less clearance between the frame and at worst there may be interference between the frame and chain. This is generally easily solved by adding an additional washer under the drive side locknut of the hub. If so, you should also re-center the axle, so equal lengths are protruding from the locknuts on both sides.

Also, you may find that you have to replace the chain. Some 6 speed chains are too wide for 7 speed spacing. Reagrdless, it is good practice to install a new chain with a new freewheel.