Be sure that you are clear that you did not do the original work and that you are using these "re-works" only as a demonstration of your skills. You don't want to appear to potential employers that you are trying to claim ownership over the original work.
If the owner of the video has a problem with you using their work, they will probably just send you a notice to stop using it. If you stop, you'll be fine. I doubt you will have any problems.
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I did this also for my portfolio, just put this : "All of the clips are the property of their respective publishers and distributors (Ubisoft, Eidos, E.A., whatever...), and are used here only for demonstration purposes" at the beginning of the video and in the description.
I never had a problem.