A 'texture' consist of up to four independent files, with the diffuse, glow, shine, and normal map being separate files, e.g:
Texturex.dds
Texturex-glow.dds
Texturex-shine.dds
Texturex-normal.dds
As far as I understand it each of these files uses exactly the same UV map, so features on each line up.
When people talk about using multiple textures they mean using multiple files of each type with different parts of the UV map on each. When I have seen multiple textures used they have generally not been the same size.
As for file formats, A DDS file is not just a compressed file, and in fact some variants are not compressed, but it is a file that the graphics hardware can use directly, while other file types such as PNG would need conversion in the graphics card driver resulting in a slower loading process and potential for additional compression artifacts due to the effect of converting graphics of one type to another.
While PNG is a very good file type for some graphics types, I would not recommend it's use at all here, as for designing the texture is is much easier to use your graphics progams native format, with layers, for designing the texture and then converting straight to the DDS format that the game uses.
Anyway, well done on getting the model unwrapped as this seems to be a major bottleneck with most models.