Huh.
Removing all non-site positions from the dwarves and adding [variable_positions:all] let dragons take over their societies. So that's all good. I haven't checked to see if fortress mode works fine yet, but it seems likely that it will. Looking at it with legends viewer, the dragons also did things like ventured into the caverns, and tamed... Voracious Cave Crawlers, Elk Birds, Rutherers, Jabberers, Blind Cave Bears, Cave Crocodiles, Giant Toads, Giant Olms, Giant Bats, Giant Rats, Giant Cave Swallows... and Cave Dragons.
So if I can embark with all that, that will be awesome. Who needs dogs when you have cave dragons?
Aww, that's a shame, you can't embark with the things they tamed. Maybe they'll be tame if you catch them - report on that to come later.
Changing the goblins to murder:shun did indeed stop dragons from taking over their civs, which I do not understand, as the human and dwarven civs don't approve of murder and they can get dragons. But hey.
Oh, I gave my dragons a couple of spheres, but they are unnecessary. With spheres, they trick civilisations into believing they are gods, but without them I am pretty sure they can still take over by force of argument (like demons and goblins).