1. 2. I use DXT3 or DXT5 both work for me. DXT1 with no alpha - kills the alpha map and the unit is totally red in game. DXT1 with 1 bit alpha loses most of the colors and the unit looks mostly black. It is a bit strange... On another computer, I am pretty sure I used DXT1 without changing anything and the alpha mask was still there...
I think it was because I saved without selecting mipmap in photoshop! Saving as DX5 with mipmap worked perfectly in the end
Here, a tutorial about the use of alpha mask on GIMP : http://forums.nba-live.com/viewtopic.php?f=138&t=80854