What glue shoud I use on Plexiglass that can be peeled off without any scratches?

Try a test spot of aquarium safe silicone on a scrap piece or an inconspicous (sp?) area.

What are you trying to do with the plexi? If you need strength and removability I don't know of any glue that is like that for plexi. If you need strength, there is silicone meant for acrylics/plexiglass, but it isn't easily removable or there is the solvent glue, that definatelly isn't removable.
Regular silicne on plexi comes off pretty easy though as it doesn't stick to the plastic very well.

Matt