I'm not Jafo who is FAR more expert in this stuff than I am. Also, I'm not a Gallery Moderator, so I guess you should regard my opinion and understanding of the rules as based on the stuff I've seen and read here on the site but not a "Pro" opinion.
Brushes, for example, don't always say anything about their use in commercial work, some are not to be used, others suggest to write and ask. Are our skins commercial in the first place? I mean, we don't sell them after all. To what degree must a photo or a texture, for example, be modified before it becomes our work? If I do a Google images search for something, and find an image that has no specified permissions, what thoughts do you all have about cutting little bits from it and using them in your work?
1. Brushes? Best policy is to cite them - which you used and by whom/url.
2. Are the skins commercial? Well, they're not for sale (unless they are Master Skins or Premium Skins), but they are available as an advantage and an example of what the software does if you get the software... and they can be displayed on the site so they are in a way commercial.
3. To what degree? The image must be 100% of your making. 0% someone else's. That includes textures, etc. As for googling? Just because you "think" an image is "free" doesn't mean it is. It could have been taken. You should assume that unless specifically stated by the artist that an image is free for taking, reproducing and distributing commercially, then it is not nor is any part of it. If you use someone else's graphics, you must provide permission from that person in the zip file you submitted, usually as a Permission.txt .
For example, many of my color picks in the skin I'm working on come almost directly (I don't use an eyedropper) from a classic Jaguar interior.
"Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read." What "inspires" you? Doesn't matter, and no one owns a color or geometric shape. However, if they are identical to something clipped from an ad or another place (not your own) and are not "fan art", then there's a problem.