I'm not 100% sure about this, but if I want to use other people's clips I contact them for permission to be sure I won't face future problems. I've used other clips before and have always got email permission so I have it in writing.
Yes, you will get strike for that. I've done this before in my other account, good thing I tried it first before I post it in my main account. There is one rule I always kept it mind is this: if it is not your video, that is already copyrighted unless you've got permission to the owner of that video.