You're right Ki does exist, it exists as Light/Darkness but the light has three different aspects which can only be wielded separately. Now as others have suggested Gokus powers use several of the aspects at once. so he is now forced to use one aspect separate of the others, which he cannot; at least straight away. Plus he would then be constrained to using one the 9 subclasses as the Light in Destiny apparently only works in these ways.(or our Guardians aren't very creative.) Also I never once said they don't draw it from the same source, just that it can only be wielded separate from each other.