I still lurk here!
The problem is not destroying the reflective layer, you do not need it anymore, you are trying to remove it completely. The problem is, it can be hard to remove it perfectly (No adhesive left behind) without hurting the LCD ribbon cable. If you remove the polarizer+reflective layer, though, it usually leaves behind a perfectly smooth glass surface, with maybe one or two chunks of adhesive. Some gameboys are different than others, though, I had one where the reflective layer just pulled right off! Try to remove the most exposed corner of the reflective layer first, and if you see it leaving behind sticky stuff, you might want to try taking off the whole polarizer.
And yes, you can use the polarizer sheets from a DS LCD, if you can manage to remove them without hurting them too much. There are two per LCD, front and back of the panel, so you get a few tries. Pulling those off is good practice for the pocket LCD, too! Once you get them off, remove the adhesive that is left over with water+Q-Tips or even just fingers. Hold it under running water from a faucet while you do it, does not hurt the polarizer.
The main thing I can say is this: BE CAREFUL with the LCD ribbon cable when you get it! It is very, very delicate.