Are Shimano Claris brifters compatible with 90's Shimano 600 parts? Is Claris backwards compatible with older shimano 600-tricolor? Can i just switch ill working parts with Claris since they are both 8 speed groups?
Yes, all Shimano 8-speed shifters and rear derailleurs except Dura-Ace are compatible. Road and off road front derailleurs have different cable pull, but since both 600 and Claris are road component series, they are compatible too.