They will only fit if you get 20mm 5x100-5x112 adaptors.
Monzas are 7.5x17 5x112 ET51, ET31 is pretty much level with the arches on a Polo, so if you're going to lower it, don't go too far!
ET31 isn't that bad, if you fitted MK1 TT wheels they'd stick out the same as they're ET31-33 IIRC.