Assimilators, no. Spreaders, not for the lowest tier, yes for two higher tiers (I do not recall if r3 has hidden spreaders, team version is r6 right now ).
The rabid guardian is a hidden nationality unit so it can be attacked by friendlies and can attack them with the ambushes. It is a little weaker than other fort commanders, but has a chance to spread rabies and disease in combat or via ambush.
Currently I am writing an AI package for the team version so the computer will know what to do with them and they will no longer be a human only race.
Yes, r3 has hidden spreaders, digitus and pollux(?), though I didn't see any xp gain from them. I also had some issues with assimilators not being able to cast any spells. Ever.