You could wear them over pants, however they would only offer you initial impact protection then, as the guards (under friction of pavement) would move. Guards over pants are CERTAINLY better than no guards. Of course, textile pants (of casual fabric) will only offer you a fraction of a seconds worth of abrasive resistance anyway, so it's kind of a moot point. My advice for you situation would be to buy a set of cheap, 600 denier or higher, overpants that zip up the sides. Then ditch the cheap armor that comes in them. That way you can wear your work/casual clothes, put the knee strap on guards OVER them, and then slip into the abrasive resistant overpants. That way, you'll still be well covered and armored. Also, if you're worried about the bulk of knee armor...I'd suggest KNOX "cross knee" armor. I swear by those things and will wear NOTHING else. They are CE approved, flexible, fit slender legs (or larger as well) VERY well! If you want to see a video of them, check out
http://stores.sportbiketrackgear.com/Detail.bok?no=4152 I use the short version (knee only), as I'm not interested in the shin portion (large).