The plug that you don't have connected needs to be plugged into a banana cable, just like your other battery. You have the balance cable plugged in, but you also need the other cable plugged in. I made a cable for mine.
I don't understand what you are asking about one leg and two legs. The SCX24 LiPo is just like every other 2s LiPo. You have the discharge lead and the balance lead.
The charge rate for 1C charging would be .35A. You move the decimal 3 places to the left. That goes for any battery. 6500mAh 1C...
Amazon. https://www.amazon.com/dp/B000BQ4XPQ/ref=cm_sw_r_cp_apan_glt_fabc_8RGW3DMZZFGZ1AX5VXJ2
https://www.amazon.com/dp/B0006O5046/ref=cm_sw_r_cp_apan_glt_fabc_KZCRNZBWZF7S0ZN527AM
I have the hex .050" driver and I have the other 3 Dynamite hex drivers and love them...
This is all I have needed. The .050" hex driver is a 1.27mm if you have a metric set. The wheel nuts are 5mm. If you upgrade to the wheel weights with the smaller bolt that holds the wheel on, mine are 4mm.