Main texture - optionally, if transparency is needed, but that requires also the transparent poly's to be at the top of polygon/point hierarchy in the model, to ensure depth sorting works correctly... (or some such, thats the last explanation i know is sorta valid, altough i never used transparencies in fso)
Glow - completely unnecessary, and only adds size to the file
Shine - optionally, altough its a good idea to include one, as it controls how much the enviroment map reflects in parts.
0 = black = no env map shows
255 = white = env map shows completely, aka mirror effect
Normal - neccesary, since fso reads data from the green and alpha channels. red and blue are ignored.
Creation of fso-compatible normal maps, if going from a bluish one, is as follows select the red channel, copypasta it into the alpha channel, zero out (aka black fill) red and blue.
There are arguments presented by Zacam as to how does this affect normal map quality, and it has been confirmed by Herra Tohtori that there is a visually quantifiable difference between zeroed out red and blue channels and non-zeroed out. I forgot what the concensus was however, which one results in higher quality...
If you are starting from a heightmap, use XNormal or a part of nvidia photoshop plugin to generate a bluish normal map from the heightmap (i'm just calling them bluish)
ALSO, beware of bulk converters like the infamous "DDS converter", they tend to work most of the time, but when alpha maps are in the play, proper results cannot be guaranteed.
nVidia's texture tools are the way to go for proper conversion of textures from tga's (or whatever) to DDS.
And, if you channel manage yourself, you shouldnt use the DXT5nm preset under ANY circumstance.
tl;dr Know your tools and how to use them.