If I show only 1 of the skins, would the megabytes in memory include the other 10 as well, also would it include all the bones, vertex placement etc of the other skins or only the single skin I would like rendered?
Regarding textures, unfortunately by default all skin textures will be loaded, as Unity in general loads any resources that are referenced in the active scenes.
Regarding vertices and bones: bone and vertex locations are just computed when active, so nothing unused will be updated.
I'm worried because I need to export in 4096 x 4096 instead of the average 2048 x 2048 I usually export since this asset is huge. The file is 20mb, would the entire 20mb run when only 1 of the skins is showing?
Do you mean that your 4096 x 4096 atlas texture contains all 10 skins? If yes, then Unity will always load the whole 4k texture and use it as a whole, regardless of how much of it you actually need. Unity or any game engine will in general load entire textures, not parts of it.
However, the spine-unity runtime offers functionality to repack skins or attachments at runtime to a smaller atlas texture. You may want to check out the Spine Examples/Other Examples/Mix and Match and Mix and Match Equip example scenes on how to do this.
