From what I've found I should be able to overlay a transparent background video clip on a background image but I'm not sure if that backround can be another video instead of another image.

Yes, it can be another video. Just use two tracks and composite.

Thanks very much, that's great news cause I got a huge u-tube project lined up!

I'll risk with an addendum, can the chroma-keyed clip be smaller than the background one (I may have painted myself into a corner, but could redo it if necessary).


Also how do i subscribe to get email alerts when someone responds (I almost missed your reply)

"can the chroma-keyed clip be smaller than the background one"

Yes it can. You can combine chroma key and rotoscoping as described here and in the video embeded there

http://userbase.kde.org/Kdenlive/Manual/Effects/Alpha_manipulation/Blue_Screen#Tutorial_2