Hasta La Motion!

I want to practice some fx on existing footages say from existing video clips or movies for non commercial use. It could end up on my reel but I’ll supply a before and after as well as credits. If I use it for a non commercial purpose is it acceptable and allowed? Or am I taking the risk of being sued or being asked to remove an fx piece I did from my reel.

*edit*
Just to clerify, I’m not saying I’ll take a piece from a huge mega VFX movie and add a small fx and cliam I did the shot, I’m saying I’ll take a short clip from a old or a regular movie that has no fx in it at all and add something. IE shot extention, cg building, sky replacement, etc.