The engine can use them all in a single layer, It will make a cube map with 6 layer named a specific way.
Here is what wiki has for each compression codec.
http://en.wikipedia.org/wiki/S3_Texture_Compression
Don't know if this is what your looking for.
FPSC seems to like the dxt1 and dxt5 flavors, also I have seen TGC use the AGBR8 under the uncompressed options.
I myself have also used the dxt3 codec with no issues.
Normals for FPSCR are x - right and y - down.
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