The biggest issue with the battery weight is the way it will change the balance of the plane. Unless the battery is located exactly on the center of gravity, you're going to wind up shifting the CG when you change batteries. If you don't add weight on the other end to bring it back into balance, you'll find that your flight characteristics change a lot as will your elevator trim.

The plug is always called a balance plug on lipo or life batteries. It's true some chargers charge through the balance plug, but it's still called that to differentiate it from the main connector coming from the battery.

Congrats on the first flight. I'm glad it went well and that you have someone to help you in the initial phase of learning to fly. The hobby is an absolute blast once you get into it.