7 will work with 6 speed.
Those old SRAM MRX200 will crap out soon. Wrk101 and I find the shifters below to be great value and foolproof, not to mention easier for the rider to make a gear selection. I just put them on an old Giant Boulder guest bike and am delighted in the ease of shifting.
https://www.amazon.com/Shimano-SL-TX...PZRGQWD6M6TPVH