tl;dr version of what bunny said

If it is a small or medium change, it HAS to be posted on the same submission.

If there is a MAJOR change that would constitute it being an actually different skin, it can be uploaded separately.

This is from the rules.
7.3.5.7Recolored or Modified Skins
Sometimes, a skinner will create several different variations of a skin, perhaps to show a character on several different teams or with several different color styles. This includes, but is not limited to, removing a small portion of skin, adding a piece of clothing or equipment, changing a shirt, and any similar practice. All skins that are created this way must be posted under the same submission, and any skins found breaking this rule will be disabled so that the author can move the skin to a primary submission before being deleted. To add variations on your skin, load the skin variations to an image sharing program and place the links to those skins in your description.