Well I have only a humble knowledge of modding and editing, but I do seem to have figured out downloading/using a mod. But anways, is it possible to download someone's unit skins, and use them without loading a mod each time I play?
thanks in advanced!:)

No... to use skins, they either need to be modded into the game or you need to mod them into the base-files (not recomended)... but either way, you'd need to change XML files as-in doing a mod.

There IS a way to auto-load a mod whenever you start Civ4 so when you load a game you also load a mod at the same time. I do this with my own Wolfshanze Mod.

It is possible thou, to drop a skin or nif into Custom Assets/Art/Unit/[wahtever unit] - if it is called the same as a existing file - unit/mesh/texture it will overwrite this unit, so you will be seeing the skin without any gameplay chenge... Basicly it's the same as just overwriting the unit's art in the game folder, just lets you keep the original file...

Let's say you downloaded a file galled HUGE_gun.nif (mesh) and HUGE_gun.dds (Texture) and want you Artillery look HUGE... Rename the HUGE_gun.NIF to Artillery.nif (dont rename the texture) and drop the Nif and the Dds files into (Make the Artillery folder, if it's missing...):

MyGames/Beyond The Sword/Custom Assets/Art/Units/Artillery

Let's say you downloaded a file galled HUGE_gun.nif (mesh) and HUGE_gun.dds (Texture) and want you Artillery look HUGE... Rename the HUGE_gun.NIF to Artillery.nif (dont rename the texture) and drop the Nif and the Dds files into (Make the Artillery folder, if it's missing...):
Actually this often doesn't work as renaming a NIF file can often cause problems.

If you want to change graphics, it's best to change the XML and it's best to do it in a mod.

Sometimes the CORRECT answer is not always what you want to hear.

hmmm ok....but is there a tutorial out there that would instruct me on how to put multiple reskins in a mod?

hmmm ok....but is there a tutorial out there that would instruct me on how to put multiple reskins in a mod?
You might want to start by clicking here (http://forums.civfanatics.com/showthread.php?t=176460).

I warn you... once you get started you may get addicted... I only originally wanted to swap-out one or two unit graphics... now I have a full-blown mod on the mod download page.

if you make alterations to the original code... and you goof.. you have to uninstall and re-install or fix your goof

Actually this often doesn't work as renaming a NIF file can often cause problems.I've never had any problems like that. Ever.
If you want to change graphics, it's best to change the XML and it's best to do it in a mod.And that can fit snugly in CustomAssets, too, you know. Just put the files in the right place under CustomAssets/XML.