This race has absolutely nothing to do with Super Buu. All of the facts you quotes you spoke are true for Super Buu, but Kid Buu was just "rage incarnate" and Fat Buu was "a man sized toddler", neither of which were very bright.
This race actually takes place in the expanded universe of Dragonball when Mr. Buu was lonely after looking at a few of Mr. Satan's magazines and actually made a mate for himself named Boobie (or Miss Buu, depending on the translation). These are the offspring of the two. All of their spawn either resemble Fat Buu or Kid Buu (only they have actual genders).
I'm sorry, you want to make a Majin Buu-themed race but you don't want to include the Buu variant that got the most screentime or that most people think of when they want to play as one? Seems silly. Super Buu's the best variant of Buu IMO, and the most playable.
Fat Buu is still pretty smart, he's just immature.
And the Majin race? Nothing actually stops them from being intelligent - in fact, a couple of games they're in they're just as smart as everyone else.
Personally I think they'd work best as a race with very few racial abilities but regeneration or fast healing as their main reason to paly one by itself, maybe with some penalties to stats... And then a racial paragon class going from 5-20 levels, somewhere in that range (PRetty broad range but I'm not entirely sure what you're aiming for). The paragon class would give them the ability to absorb people, eventually bump their fast healing up to regeneration, and as they level up or after a certain number of levels and HD (Assuming it's a low nubmer of levels) gain more racial abilities. The class itself, if 1-20, might actually have quite a few dead-ish levels that are largely so that you can store class abilities from other classes, along with possibly a few racial abilities or at least boosts to their ability scores (Kid Buu got stronger from absorbing the super muscular Kaio, for example, and even if you don't like him Super Buu progressively got smarter the more people he absorbed too. Especially when he absorbed Piccolo, though.)