This is actually my first post (I had been considering posting the details of my first truly planted tank soon). I'm still seeing a few fine particles from the replacement media I laid down in my tank four days ago (peat/sand covered with Eco-complete), and I'm very glad to see this thread. I may throw in a poly layer in my filter to see if it helps.
The only thing I'd be concerned about the media is the ability of plastics (polyethylene, polypro, etc.) to house bacteria (just to back this up, I have a biochem degree and years of lab work). It should be great as a mechanical filter, but it won't have the surface area to be great biologically, so taking out a huge percentage of your other filter media for it might be counterproductive if you don't test often afterwards. Squishing it down should do nothing but help filter, as long as it doesn't significantly hinder your filter flow rate.
I quickly decided in this hobby that you're almost always better off using a bit of one thing to do one job, and a bit of another to do the other job...rather than trying to compromise with a jack of all trades method. Balance is probably the biggest lesson fish have taught me, and I truly appreciate it.
