Welcome to the RWA forum!
I recommend you follow the instructions and you should be fine. Wear eye protection!
Do one battery at a time and do not short out the wires, and do not mix up the wires. Also, don't keep the soldering iron tip on the battery terminal for longer than a couple of seconds.
If you are not comfortable with soldering, I do not recommend doing it because you could get into trouble and repairs can get expensive.
You can send to me, or take it to a local electronics shop that is familiar with soldering/desoldering.
Regarding speakers, try both and see what you prefer. They both should work fine.