Hey there! Thank you for pointing out the awkward wording of Rule #6. We appreciate that and have fixed the wording to be clearly, we hope: "6. You are only allowed to use the base assets provided by the base purchase of RPG Maker MZ (no DLC or preorder bonuses), or anything created by yourself (or someone on your team). You are allowed to customize the base assets. You are also allowed to use any freely distributed plugins. Bottom Line: If you have to pay money for it, you cannot use it."
As for Public Domain things, those are not things that you created. You are taking something else and modifying it. So no, those wouldn't be allowed. We'll get back to you on frankensteining something (taking a piece here and a piece there), but for now, assume that those aren't allowed either until we say differently.
We'll work on getting that default list. Our apologies, as we hadn't realizes that the DLC stuff would cause that issue.
You are completely correct on any plugins that cost money (VisuStella or otherwise). If you have to buy it, you cannot use it.
And lastly, you are correct about no MV plguins that happen to work with MZ. It needs to be for MZ. Thanks for bringing that up!
Please let us know if you have any other questions!