Dont fear the 8g wire- I've seen someone fit their traxxas plugs fine, just requires a careful bit of trimming on the insulation (the wire itself will fit fine, just trim the insulation flush with the connector housing). As for the hole in the battery compartment for the wire to exit- a dremel cures all ills
Have no fear over minor modifications to the chassis like that, it wont hurt anything, nor will it look ugly if you have a steady hand and take your time with the dremel, or even some small hand files.