If I connect a battery to the receiver..
 

One thing I've been wondering about, but afraid to try, is this;

Let's say have 2 lipos in series connected to the ESC, it's turned on, then I plug it in the receiver, which has a battery plugged in the battery port. What will happen? Will this work like a BEC?

if you a have BEC built in the ESC you Should have No problem at all

I'm not sure if this is what you're saying, but if you are using an rx battery, then you disconnect the red lead from the esc (with built-in bec) to the rx. You don't want 2 power sources coming to the rx.

Yeah, I meant plugging in an RX battery to the receiver while the ESC is turned on. I will just unplug the red wire then. Thanks! :)