the hybrid fear comes from the media
Back recently when they were trying to enlarge the florida issue with burmese pythons breeding down in the glades, they talked about them being able to find and breed with the African Rocks down there to form what they called a super snake basically. The Hybrids have been done, and they are just as nice as the Burmese are, just needs proper husbandry to keep them nice. Think of it like a fussy kid, the kids only being fussy because there's something causing them discomfort (hunger, fever, runny nose, poop filled delivery bag, etc). As long as you meet the needs of these snakes (captive bred specimens) their docility will be there. Instinct does take over and take some time for conditioning to get a more jumpy snake to calm, but they calm none the less.
It's already been said but since I'm posting on this thread anyways
Green (Common) Anaconda is the heaviest growing snake
Reticulated Python is the Longest growing snake
Burmese, have potential to be both and has been in the record books for length (the one died not too far back)