Can I use Shimano 8 speed rear deraileur (Claris) with 6 speed SIS downtube shifter (for a 6 speed freewheel)? So currently I have Shimano SIS downtube shifter (6 speed) with indexing, a 6 speed freewheel, and Tourney TX for the rear deraileur, they index properly.

If I want to swap the RD with Shimano Claris, will it still index properly?

Why? because I'm planning to upgrade the wheelset and shifter in the future to end up with full Claris, but I want to do it incrementally.
Proper indexing of the derailleur cage relies on two things:


Length of cable pulled per click of the shifter.
Derailleur actuation ratio (how many mm the derailleur cage moves per mm of cable pull).


If the derailleur is swapped for another with the same actuation ratio the indexing will be unchanged.

Shimano used an actuation ratio of 1.7 from 6 speed to 9 speed mountain and 10 speed road. (Source: Arts Cyclery Blog - Drivetrain Compatibility).

So yes, the 8 speed Claris derailleur should work with the 6 speed shifter and 6 speed freewheel.