I have been playing it as the skinks get stomped. Usually by skaven rat ogres. But, the other day, I got a message from the Skaven player. He told me that on a podcast they were talking about this and a mixed unit cannot be stomped. Not sure about the thunderstomp. Here is the link to the pod cast. I haven't had a chance to listen to it yet. http://heelanhammer.com/category/episodes
Thanks for the link, Arli; just listened to it. I guess if that's how they're playing it in the big leagues, I'll go with it. It still seems weird basing the unit type for a lizardman unit off an FAQ about a Skaven spell, but oh well...