With regards to keeping the pinions on, I used to use loctite and they were forever coming off. I switched to 2 part Araldite and haven't had a problem since... best to rough up the shaft a bit first; I used some wire cutters to basically knurl it.

I found that the weight benefit of brushless only really works if the drive runs off the same battery as the weapon... otherwise fitting a second battery wastes most of the weight gain.