The LiPo's should weigh less than those 8 cell NiMh's.....I am not sure on the ESC using both batteries. I think you can do it, but the ESC should have a certain low voltage cut-off programmed for the LiPo's, 4.2 volts I think and I believe that sometimes causes problems with the NiMh's that usually have an ESC low voltage cut-off programmed around 3.2 volts. I have never tried running both on one ESC. If you get an ESC that you can program the low voltage cut-off on, you can change it per the battery you are using and I believe everything would be fine.