Posting the same Q in several places is a no-no.
That said, to address your new questions:
MA = fail, just say no.
6s lipos will not fit, as even with the very low mah cells they are just too tall for the trays- the principle of usinf a couple lower mah packs in parallel is fine though.
Saddly, the traxxas monkies/engineers failed to take this into account really, so your bet bet for a 6s setup is a 5000mah 3s lipo in each side of the Erevo, just cant fit anything bigger really unless you hack the top off of each battery compartment.
A123 cells wont fit period, not without removing the doors, and even then you can only fit 5 in each side (side by side like a nimh pack), so that onyl nets you a 4s lipo equivalent setup.
Read my updated Erevo FAQ for more info and ideas, and a diagram of the battery compartment dimensions so that you know how much, or little, space you have to fit the lipos into.
http://www.robotwars.00server.com/erevofaq.html