Some MM's are more tolerant to higher voltage than others. Some have been able to use 5s lipo with no problem, while others may not even arm. 5s A123, even "peaked", is pretty darn close to 4s lipo "peaked", but still hard to say if a particular MM will work. Even then, the 1518 can draw some hefty current, so I would definitely use punch or torque control to help limit the amp spikes.
And yes, disabling the BEC is just a matter of pulling the red wire out of the throttle cable (I pull the connector out of the housing, fold that wire back, and heatshrink it to the rest of the cable) and using an external BEC. It's pretty easy, but
this may help.