no, tiff and tga for everything
If you want transparency in your glow then just make sure it has the transparency when you render it. Look at the alpha channel it should be white where it's opaque and black where it's transparent. So the glow should be tones of grey.
In order to get the glow around the cube you just need to make sure that the image you render is laid on top of the other one. It's as simple as that. What are you using to put it together?
Technical Director - Framestore
Currently working on: Your Highness