Regens and shields do stack for Neutral Sect.

The only things that don't stack are:
The same regens/shields, so two Nocturnal Benefics, or two Diurnal Benefics, if you have two ASTs. (WHM regens however do stack)
AST's Nocturnal Shields and Scholar's Galvanise shields.

Neutral Sect Shields do stack with SCH's, and Catalyse does stack with all of AST's shields.