Balance plug is used by a balancing charger or an external balancer to balance voltage in the individual lipo's cells. Lipo can be damaged by overcharging or overdischarging. If cells start balanced you're less likely to overdischarge a cell relative to the others. I've used lipos both with and without and the ones with a balance connector last longer since they can be balanced.

The C is the lipo's discharge rating. A amount of amps a lipo can discharge is its capacity (mah)/1000 x C rating. So a 2000mah 10C lipo can discharge 2000/1000 x 10 = 20amps. As long as the lipo you pick can supply your servos, its fine. For your use, C rating is not paramount like it wouild be if it was teh power source for an electric MT or TC.

Here's a little reading about lipo -

http://www.rchelicopterfun.com/rc-lipo-batteries.html