There are different DDS subformats. The smallest file size with DDS has no alpha channel and good compression. Some other subformats do have alpha channels with several qualities (amount of bits per pixel).
So DDS could be the all-in one solution and you don't need bmp, tga or pcx in the future.
DDS don't has to be square. Only if you want to create mip maps automtatically then they need squared sizes. DDS is very flexible. Just play a bit with the GIMP plug-in and you will see.