I know the power of an enchantment depends on your enchanting skill and the size of the soulgem but does it also depend on your skill that you are enhancing? If I had 100% archery skill could I make more powerful fortify archery armor than if I had 50% archery skill at the time?

No.

Nope you can wear armor and a ring with enchanting %(whatever the % is) better on it and use enchanting potion then you can make a powerful enchantment on something.