Felted Furry Foot Warmers?

I have recently knitted up a pair of Felted Furry Foot Warmers from Debbie Stoller's "S'n'B Nation", and I'm not sure how much they will shrink when I felt them. Does anyone know? the yarn I used was Alpaca and wool. Also on the top part of the slipper I've knitted in a 100% polyester eyelash yarn. Will that stop the felting? Any advice would be great! (Also I have done my gauge swatch, but because I live in Australia and we are in a drought and we have to save water, I can't repeatedly felt the thing if it's only a 10cm square. So I don't need the usual "Do a guage swatch to find out" advice. Thanks muchly!)