When using a video clip, you have some issues. File size, and resolution. Bootanimations should be under 4mb. They SHOULD be under 1-2mb, but you can get away with it sometimes.
A 20 second video clip broken down into .png files could be anywhere from 20 files to 400+ files, depending on how smooth you want the bootanimation. More files, larger bootanimation, slower boot.
Honestly I've done a few video bootanimations and I don't recommend them.
Also unless you wanna watch your phone boot up in landscape mode, it's gonna be tough finding a clip that will look good in portrait mode. You will need to resize the video which will look like butt.
Just some thoughts.