Can I use Shimano 8 speed rear deraileur (Claris) with 6 speed SIS downtube shifter (for a 6 speed freewheel)?
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.