I would use 5mm on an 8th scale for sure.

IMO Bullets = resistance = Not recommended

sometimes you dont have a choice though. The tekin has tabs so you can directly solder, but some motors come with the wires exiting the case of the motor, and in the case of the VTX8 it has wires as well. no choice there but to use some method to connect the two.

With the copperhead R speed control can I up the voltage to 6v using my proguage in order to power my Savox SC-1256TG High Torque servo or should I keep it at the stock 4v?

stock 4 volts?
i don't know much about the Copperhead ESC, but i know you can adjust the BEC voltage with the ProGuage on the VTX10 and 10R. you can set it even higher if you want to, up to 7volts.

I can adjust it but havent tried because i didnt want to mess anything up. The specs on the speed control say up to 6v

6v is fine

Thanks guys, my main reason is ease of use. I want to be able to separate the ESC and motor when working on the car, rather than risking the solder connections going bad and redoing them over and over.

I guess the bigger the better? The ESC wires are under 3mm, I just measured them.

5mm should be fine. no real need to go bigger.

I have some spare 5.5mm bullets I no longer have a use for, I may use those. Initially I was thinking 4mm.

I am sure the 5.5 will work fine. They will hold a little bit better then the 4mm also.

Thanks, I'll use those.

I just picked up a Dex210 and was thinking of getting the VTX10R+motor+programmer. I have yet to decide on which motor but have it narrowed down to either the 13.5 or 10.5. I'm just looking for any help on where I should start with gearing should i go with either. I have the stock spur and 26,24,22,20,18, and 16 tooth pinions.

you dont need an R esc for 10.5 or 13.5. The vtx10 with PG is plenty.

I would get the 13.5, get out and practice with it.
you can always throw some boost to it if you want more power/speed.
if you do run boost, then the 10R is not a bad idea.
the R is not needed, but it is just nice to have the R if you don't want to worry about heat. they seem to hold value a little better too.