Can't comment on the mixed weapons, but I've seen others run that and it seems to work well. I agree on moving the ammo. I always fill the legs first and mix it up, never carry all of one kind in the same spot.
Re: back armor...I don't have a single mech with more than 18 on the back. Rarely ever taken out from the rear just the same.

I need to try some new builds on my Firebrand. The ac40 isn't as tough as you'd think. It stands out like an Atlas and people come running over to take you out. LOL Still fun, but attracts too much attention.

The atlas looks good Feet, just hot! Comes with the territory though I guess.