The texture on the base model is split into different areas (i.e. legs, "vest body", hands, short sleeves, long sleeves, head) which can refer to different textures, so the clothing could be from one texture (i.e. armour underclothes) and the head and hands (or arms, if relevant) can refer to a separate texture i.e. from skin colours. The additional parts of armour (that you have modelled) have their own texture(s) too.
In theory, as long as the "edges" finished in the right place, you could use the legs off one texture, the shirt off another, and the sleeves off another.
Hopefully that makes sense. If you download a few of the existing models (they're scattered around the 3D Development board) it should be fairly clear - basically, the head and arms are all "default white guy", but could actually use any of the other skin tones instead.
There's a question I asked (number 3)
here about this, and if you look at the following answers from Karpov, it should make sense I think.