What thickness of plastic or plexiglass do I need to comply with building code in California?

You need the code from your building inspector, not the whole state, even if most towns use the state code. There may be stricter codes for rails in lofts because of the height and hard floor below, which could result in death if someone fell. And it wouldn't hurt to exceed the requirements.

Lumber and manufactured products for guard rails have to provide load/tensile strength info, so you need to research if it's an atypical plexiglass rail. Be sure to ask both the inspector and the manufacturer about fastener ability and requirements too. (Aside from the height, posts, and balusters or panels and spacing.) Since plexiglass at right angles is usually glued, can it withstand metal screws and will any be angled?