I surface the saddle bottom by sanding it on appropriately course sandpaper held flat on a thick piece of Corian. (I used to use plate glass until I busted it.) I check the flatness with a machinist's square. I also make sure the saddle slot is flat and correct it with a homemade scraper.

If the saddle needs to be higher I have sometimes superglued a piece of bone, Corian, or Tusq (the 3 materials I use) to the original and resurfaced the bottom at the correct height. Shims eat tone in my opinion, but my method seems to work okay.

An one piece saddle is fine if it's got the right intonation offset and/or has the top edge shaped to intonate better.

I'm not sure of the physics of the thing but I fail to see or hear how undersaddle piezos are better than the soundboard/bridgeplate variety, so I prefer not to have an undersaddle pickup....but to each his own.

Peace, Mooh.