Considering how Ruin and Preservation interact with people, i.e. the power of Preservation immediately fills cracks in the spiritweb, stopping Preservation from inserting thoughts, maybe burning Lerasium repairs any damage to a spiritweb, and the patch material is what gives allomancy, I'd be willing to wager that the only requirement for burning Lerasium would be that your spiritweb can't be in perfect condition, and the more damaged the spiritweb, the more powerful the resulting allomancer.
As for a Lerasium spike, I can't imagine what it would steal (if it could steal something in the first place), but I'm going to say that receiving a charged Lerasium spike wouldn't leave holes in your spiritweb like a regular hemalurgical spike would.