If you look at the 8.4v motor they list 9100 RPM loaded. This is rated running 9.6v. Running only 6v will be about 5500RPM. I believe this is about where you would like to be.

Current in amps is not critical at all. The motor will draw as many amps as it wants depending on what prop you toss at it. A smaller prop decreases current draw a larger prop increases draw.

Ryan