I think the founders could shape shift into a borg, and odo explicity mentions that internal sensors can't detect his people unless they are in their founder birthday suits, he states if a founder is a rock you'll detect a rock, if a founder is a humanoid you'll detect a humanoid. With that said we see the founders actually become what they shap shift into. If the borg assimilated a founder in humanoid form then it would be like assimilating any other humanoid. At that point you would have an assimilated founder. Also at that point the borg then know everything about them and can adjust their nanoprobes accordingly to be able to assimilate them no matter what shape.
The Vorta...i agree with whats already been mentioned...
The Jem Hedar are interesting...Once a human is assimilated (although I don't know if it specifically mentioned in Voyager with 7 of 9 becoming accustomed to eating) it appears that natural consumption of food is no longer needed. The Jem hedar can't survive without the white much as human can't survive without food. I wonder if borg implants can and would be used to replace the parts of the anatomy that cause the genetically engineered addiction cirumventing it altogether.
I don't know...just guessing...excellent conversation though!!