The Cruel AIs do cheat when it comes to resources and credits and I think maybe on research. Not sure on the research one.
Cruel and Vicious do get an AI research bonus.
Not sure where they get the credits, does the AI cheat?
Heck yeah. Pretty much all RTS AIs do at advanced levels. It just doesn't become too obvious until cruel because they spend so many resources on stupid things.
If you mean you have them on locked teams against you, yeah that won't be a fun battle. Its still quite possible because even with the extra resources you can expand faster than they can and the AI has no clue of how to defeat highly fortified positions. Given enough time you can wear them down, but I wouldn't have much fun doing it.