Scarlet -- Yikes to the slippery add-on!
You know, I think I misspoke when I said I had a plastic sole put on...after reading your comment I gave the sole the touch test tonight, and I think the shop actually used rubber -- but a thin, hard rubber vs the thicker, more sneaker-like rubber sole I've also had replaced on boots.
Kind of like the one here:
http://www.santanacreative.com/women.html
I certainly love my treaded Vibram-soled boots -- they are nice and cushion-y for one thing -- but my skinny-soled adjustments do the job.
Since you have your eye set on a specific pair of boots, maybe check around and see if there's an alternative to that slippery sole guard you got?