You’re correct, the process for alpha video using the Designer tool is outlined here.
The Studio implementation is a little different due to the fact that only .OGV videos can be imported into a Studio project.
As such the current process for alpha video within Studio is as follows:
Export two videos - the color video and a mask video (black and white) both as OGV files. Make sure they both have power-of-two dimensions in width and height. Recommend 256x256 for both videos for performance reasons.
Drag both into Media Library.
Create two VideoTexture nodes:
One set source to the color video.
Other set source to the mask video and tick the mask property.
Create a Material node:
Set the skin to the first VideoTexture (the color one)
Set the mask to the second VideoTexture (the mask one)
Create a plane object. Set the material property to the material we’ve created.
In an event call restart on both videotexture nodes at the same time to have them play in tandem.
This implementation is due a rehaul but is the current way of getting alpha video in a Studio project.
To answer your other question you can see Designer supported formats here and Studio supported formats here.
Mark