If you use a good 1400-200 LIFE 2s pack you can use it for both receiver and glow.
Reversing with these glow drivers is required no matter what radio because the Chinese designers did not understand that we want glow assist on low throttle and off on high throttle.
HK also sells a servo reverser so if you had a Y off the throttle you can have one branch to throttle servo and the other to servo reverser and to onboard glow.
Servo reversers are cheap and work well.

Note I would caution against a 2s LIPO as the "glow" may be a bit hot!!(oops I see you say with a BEC)

These onboard glow only draw about .8 amps at 6.6V because they knock it down to 1.2 or so for the plug so I am surprised your battery would go low. Using a reasonable LIFE pack voltage drops from 6.6 to 6.4 typically.