The brake/shifters you linked to are a useful way to change gear without taking your hands off. Pushing the lever sideways will change one way, a small lever changes the other way. Unfortunately I don't think you can get them in 6-speed, and the cable pull is different for 9-speed so these won't work. The closest you'll be able to use are stem shifters or mountain bike-style thumb shifters if they're compatible with your handlebar diameter.