I would set them to "Additive". Then you don't need to add alpha channels and you can get additional "glow" when they overlap eachother.
I have never used gimp, but to make the background tileable you need to offset the entire canvas by 50%, so if the background is 512x512, shift the entire image to the side 256 and up 256, and then remove the seams that appear after this shift. In photshop there is an "offset" filter, if you have to do it by hand, cut the image in half and swap it manually.
If it would help the forums, I can make a "creating simple tileable texture" tutorial.Greg2012-05-10 00:36:01