About "borrowing ideas", I'm fine with that.
This is how we move forward, there's no shame in using ideas or concepts you've seen in other skins if you feel that they really improve the experience. It's actually a good thing since that idea/concept will reach more people through your skin, be reused, and eventually become the new norm. (eg : blank sliderendcircle. This wasn't that much of a thing 2-3 years ago but is one of the best changes I've seen since)
Regarding your next point, pretty much everyone uses the same sounds in one way or another, especially hitsounds. These are hard to come by and even harder to create from scratch, only a few skinners can claim sounds as originally of their creation.
Credit : If you use or intend to use a skin element from another skin, ask the creator for permission or look for permission info in his skin post.
If he's okay with it, just go for it and mention him in the credits. Messaging him about it is always nice even if permission is given in his post.
Mentioning people who inspire you with their skinning practices, ideas and concepts is also a good thing. It warms up the community and stuff.
Also, no mixed skins. Strive to keep it >90% original in terms of image files, and you should be fine.