Look at it from my perspective okay. OKAY?!?!?!
Fat Buu + Good Buu + evil Buu right? Reverse the order of that equation, what do you get? Super Buu. Makes sense to me. See that from my perspective. its like the commutative property, you can change the order and still get the same result, only this time in the terms of power levels, not appearance. See the even so more from my perspective.
Okay, on to Kid Buu.
Like you said, Kid buu gets stronger as he fights, (Which I think he ripped off of Broly but whatever...), so initially, he would be at his base power level, right? This would mean that since Fat and Super Buu have higher power levels (Absorbed Kais, Etc.) and Kid Buu is just the base form, before all that absorbing, they could potentially kill him before he gets strong enough to decimate them. Ditto what I said perspective.
I disagree. Super Buu = evil buu (and his energy) + the raw power of fat buu. Super Buu was the result of the evil taking over. Had fat Buu won, the result wouldn't have been the same, as the passive nature of the kais (and the kais only it would appear) have negative effects on his power.
They only had higher power levels than him 5 million years ago. As I stated earlier he seemed to magically imprint the strength of his counterparts into himself. Otherwise, he wouldn't have even been able to go toe to toe with ssj 2 goku.
That's what I mean. There are essentially two kid buus. The one who was weaker than a regular ssj - 5 million years ago. And the one who can batter a ssj3 5 million years later.
So there still hasn't been any evidence that they start off with a higher power level than him. And he has just too much to put on the table. He clearly doesn't run out of energy........ever. Fat Buu clearly did. Not sure about super buu. He also has the best regeneration out of them it would seem.