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I've included non-wool yarn in felted items before (such as eyelash yarn), and everything wool felted and the non-wool stuff didn't. Basically, what happens to the non-wool yarn is it gets washed!
Hence, it stays the same (unless for some reason it's really fragile yarn and gets torn up, but I've never had that happen).
If you're really worried about it, knit a small square of half wool and half eyelash yarn, and then felt it to see if you like the end result!!