Plexiglass is not a well defined term, 2mm of certain exactly defined and approved material such as polycarbonate
is ok. I guess you mean polymethylmethacrylate (=PMMA) when you say plexiglass, not some resembling trade mark. It can be used as well in this place.
1) There should be also certain minimum route length along the surface of the insulator (=creepage). It can be for ex. 8 millimeters for 240VAC mains. The requirements vary with operating voltage, environmental conditions, mains overvoltage category level and the end application.
2) There are as well insulation requirements for the transformer.
You really should get the safety standards for mains AC operated equipment if you are building them.
BTW: is it possible to change the lengths of the spacers? Then no extra insulators would be necessary.