My 'secondary' youtube account just got monotized, I was using it to post up viral-ish videos to get some spread over to my main channel (which is also monotized). My question to BH today is how risky to my Adsense account will it be to re-upload videos from WorldStarHipHop, Nico Douga, etc? Ones that have low chances of getting copyright strikes but are clearly not my own creation. Does Google cross check that stuff?

They usually won't cross check it unless someone makes a copy write complaint about it. You could always do a review type video on the music so it's kind of your own creation or go to the owners and ask permission to display their video. If their video is up for public display then there is a good chance they won't mind if their just trying to get exposure.

Your video won't probably pass the monetize review and you will be also warned by them. You should know, you are allowed to monetize video not mean you can monetize any video. You have to ask youtube team to review the video before your ads shown on it.

I've done some similar tests with monetized accounts. The moment youtube and google detect the copycat video, it will say third party match and wont allow you to monetize the video. Then you will get an email saying its a copyrighted video.

But theres only 1 way for knowing. Try it. I use a spare account to test random vids first.