Skaa are encouraged to breed, so they can be easily disposed of without hurting the workforce. It is the Terris who's breeding is controlled. Skaa is just another word for Peasant; they are not a separate race from the nobles.
Yes, all three magic systems have references to metal in them: Hemaglobin and Metallurgy becomes Hemallurgy (Blood Metal), Ferris (Iron) and Alchemy becomes Feruchemy, and Alloy with the suffix "mancy" to make Allomancy. It is interesting that Hemallurgy seems to be the most descriptive of the three. Feruchemy and Allomancy sound like abilities. Hemallurgy, however, seems to describe the process more than the magic. Metallurgy is not just the study of metals, but also the process of combining metals to make alloys.
The number of spikes, as well as what they are made of, must be a factor. I also think they would kill the subject even if it wasn't necessary to ensure knowledge of the process was not leaked. If the Skaa learned how to create beings to challenge the Inquisitors, it would make the Lord ruler's job more difficult. Remember, this is a person who required Skaa women to be killed after Nobles bedded them, just to ensure the Skaa would not gain Allomancy, and executed Skaa nonstop for 7 hours after the Rebel army was defeated. They didn't even win the battle!