If you have an USB port on your solar system it should be already regulated to 5v.
Yes you can plug a LiPo cell to the battery charging port of the Proto. You can even put several cells in parallel, as long as the battery fully charged is around 4.2v
There is already a battery protection system on the Proto, but I personally prefer to use a battery with an extra protection circuit but it is not mandatory, but of course it is you to judge if you need it or not. We are not liable in case of a battery issue.
Seems like a cool project, feel free to share some pictures or video!