I think the answers depend a lot on your specific project.
The boat I am more familiar with (Catri) uses dihedral swept back asymetric rudder foils for lift only. They are not intended primarily for pitchpole prevention. The main foils are not swept back and the rear float foils are not dihedral. There's a picture of my boat attached. All depends on what the design requires from the foils.
It seems logical that rudder foils dedicated to pitchpole prevention should be symetrical and dihedral, but I am not sure if they must be swept back.
The geometry tells us that the angle of attack of symetrical rudder foils in a boat about to pitchpole will never exceed (roughtly): arctan [rudder foil depth / water line length].
At the design pitchpole speed this angle of attack may or may not suffice to avoid pitchpoling. Do the math for your project and you will have more elements to decide if the foil depth is enough or if it needs additional (negative) angle of attack.
