Super buu is much stronger than kid buu because he has power of Mystic gohan,Goten,Trunks,Picollo and anybody (some kai's+ power of hes kid buu form) Kid buu? he hasnt that power and he is dumbass... (super buu has inteligence of Picollo) Any questions? ;>
Buu with Mystic Gohan absorbed is Gohan Buu.
Buu with Gotenks absorbed is Gotenks Buu.
Buu with Piccolo absorbed is Piccolo Buu.
Evil Buu with Good Buu eaten is Super Buu.
EDIT: I came up with a new conclusion about what Kid Buu really is.
When Kid Buu absorbed the strongest Supreme Kai, he transformed into a Puff Buu. Then he absorbed some peaceful-weak-fat guy. Then he became weak-fat-controlable one - the Fat Buu. But the pure evil of Kid Buu still existed in Fat Buu. So when that evil got an opportunity to go outside, he used it. Evil Buu went out by taking most of the power, less body and some more brain, than Kid Buu had. Then he ate Fat Buu and reserved him some special place inside himself. He used Fat Buu to get some more body (to fit him best for fighting, moving, etc.), strenght of the strongest Supreme Kai in Fat Buu. Absorbtion is a technique, which fuses your body/mind/power with the other's. By eating a candy Buu can manage that candy the way he wants since it's small and powerless. After reverting to Kid Buu, he lost the power of the strongest Supreme Kai and a lot of brain (LOL). Anyway that Fat Buu inside him still restricted a bit of his mind, so he got rid of Fat Buu by spitting him out.
It may be that Goku has reconsidered about fighting Buu alone plus he knew that Vegeta wouldn't want to fuse again (they would still have fused at the end though). He said before the fight started that they'll try to do something if they can. Also Goku wanted to make a plan before Kid Buu finds them.
So (I didn't add appearance changes since we all know them):
Kid Buu = original.
Puff Buu = Kid Buu + strongest Supreme Kai's strenght and brain.
Fat Buu = Puff Buu + some innocent/weak/fat guy's brain and weakness.
Good Buu = Fat Buu - Kid Buu's strenght, evilness (which Fat Buu didn't use) and some of the Kai and other guy's brain.
Evil Buu = Kid Buu + some of the Fat Buu's brain.
Super Buu = Evil Buu + usefull things taken from Fat Buu.
Piccolo Buu = Super Buu + Piccolo's brain and strenght.
Gotenks Buu = Piccolo Buu + Gotenks' strenght.
Gohan Buu = Piccolo Buu + Mystic Gohan's strenght.
Good Buu = Fat Buu < Evil Buu <= Kid Buu < Puff Buu = Super Buu < Piccolo Buu < Gotenks Buu < Gohan Buu.