can someone pls ?

so ineed external bec so i can run my 12cells?
what if i run 1 pack of 4200mah 2s2p lipos?will it make a difference versus 12cells of 3800mah packs in series?

and so after i take that center red wire i can just plug my batts or i need to do something else?and will i still need the external bec?or i can put the external bec and leave the red wire connected?