Kid Boo >> Grey boo.
Read the pages, wolf. Grey Boo is absorbed by fat Boo. I think since fat Boo which still had some good in him released that, it still carried some not so pure, raw evil and had Kaioshin elements. Or maybe that's like the crude representation of Buff Boo (with South Kaioshin absorbed) but still it doesn't seem because he had to absorb Good Boo (fat Boo after losing power), but then it's also a possibility that absorbing Good Boo was simply because of Boo's powers in general, still being in Good Boo. While he may have released his anger, he still had Boo's power (tho idk if it makes really sense since Kid Boo was pure evil so pure evil should be Boo and like I said if he was influenced by just the South Kaioshin part he would be Buff Boo...
Absorbing Boo would be the explanation too if Good Boo has some South Kaioshin leftover or is South Kaioshin was within Good Boo, but with the Dai Kaioshin being the dominant part and South, not. So South and Dai were inside Good Boo... But then the same question arises of why Grey Boo wasn't Kid Boo then... So perhaps that's the rage of fat Boo which isn't pure, and at the same time, since it wasn't pure anger, Grey Boo wasn't as strong... And good boo had South and Dai absorbed, so Grey Boo being part of fat boo distorted anger needed to absorb Dai Kaioshin and South (which makes good boo) and this time control the dominant force. This could explain he needing to absorb