why wouldn't you just use a bec that does 6s?
http://www.castlecreations.com/products/ccbec.html
the motor thing would be important to prop so the max watts the motor is rated for doesn't go over. amps times volts...... theres no capacitor or anything to grenade except overloading a motor into submission from making too many watts from too big of props, burns em like a fuse. usually when a motor gets a volt rating like 4s, 6s, whatever, it is a recomendation for the marketed application. not necesarily a engineering problem...... ie, this motor is made for 12s in this heli or this is the 8s motor for that heli, etc. it's to achieve a desired head speed in this case rpm while it's lifting and the props that'll do that if are even available then how that handles carrying the load you want to fly.
the general rule is lower kv and bigger props make more torque, faster spinning smaller props make more hp, they slow less with a load
don't have any specifics for your gig, but that's basic brushless motors as i have been using them with no problems. I fly 3s motors on 4s, have been forever. my helis are all weird custom built electric conversions, they all have a different motor than what is marketed to them