The only Rayman 1 boss that turned good before the Great Protoon was returned was Bzzit/Moskito (the second Moskito you fight is a totally different one, evidenced in Rayman 1 for the PlayStation). The rest showed up "good" in the ending credits, except Mr. Stone and Mr. Sax, so I guess they just got pwned. And I'm fairly certain that Mr. Dark wouldn't fight you on his own, he seemed to like sending out all kinds of crazy crap for Rayman to deal with (curses, Bad Ray, energy balls, flames, etc.).
As for the enemy, it's pretty obvious that it will be Andre, as mentioned before, but I don't think he'll be behind the Rabbid invasion. The hand shadow scene at the end of R3 was officially confirmed to be a flashback, so he's come back some other way. What we need to try to figure out is who Rayman's "girlfriend" is, and who the "old enemy's girlfriend" is.