BIg problem fiegao shaft

Got a problem the pinion gears i got and motor shaft don't fit ang my big problem is that i'm here in manila and its really hard to find ofna gears here. You think i could have the shaft on my motor machined shoped and make thinner to fit the regular pinion gears that i bought? Also how do u series the mamba max for a two battery setup for my monster. I also setup my racer x shocks with blue and black springs with 50 oil and it still botttoms out. what should i do?