Knight

I was reading through The Alloy of Law, and when I came to the Ars Arcanum at the end, I found something a little confusing. A Feruchemist with Nicrosil stores his Investiture. Does this imply that there are instances where they normally wouldn't be able to use it, for example if someone from Scadrial traveled to Sel, or is it saying if, for example, you were a Twinborn, you could burn your metal while storing and draw on it later if you didn't have any metal? Also, on Hemalurgy. I was looking at the most common chart on what type of metals steal what, and I'm not so sure I agree on some of it, mostly with the Gold and Atium spikes. I understand that it is mostly just guess work, but I think I have it partly worked out. So far, the spikes that steal allomantic and feruchemical powers are spikes made of the alloy metals, the push metals. So bendalloy and electrum would be the metals to steal allomantic and feruchemical temporal powers, and nicrosil and duralumin should steal the enhancement powers. But Aluminum is supposed to steal allomantic enhancement, so I don't know what happens there. Where is the canon that says aluminum spikes steal enhancement powers?