Any skin you make can be used offline. As for online...it will probably be changed after the final version comes out, but this is waht I know.
-you can use your skin, but know one else can see it. You just appear as their equilelant skin. For example, joe and moe were using their custom skins. Joe was using his "p51d1" and moe was using his "p51d2." from joe's point of view, moe was flying with his p51d1 skin, and from moe's view joe was flying with moe's p51d1.
let me clarify: this will be changed when skin uploader is implemented, but currently if a person is using a custom skin whatever skin they are using will appear to their equililent to the skin (two different pilot's p51d1 skins will appear differently to the pilots to each other)
confusing i know..