I think the setup from Mo I saw had 2 carbon and 2 aluminum shoes. So as matt said some people prefer the engagement of the carbons,
as far as the aluminum and Losi springs Mo probably had extra losi stuff and since they interchange might as well use it up.

Yes the Serpent bells fit, but there is no compelling reason to use them unless
you want to use 5 x 10 bearings.

I suggest you pick up a package of these to help with shimming if your gonna
try other bells. They go in between the nose of the crank and the step
washer.

http://www.amain.com/rc-cars/kyosho-...kyo96641/p3868