I'm thinking it is something to do with the [SLOW_LEARNER] token, actually; it seems to be one of those tokens that has a collection of functions. It seems to imply [INTELLIGENT] to some degree, but it also makes the race count as a pet in many ways and is not an acceptable tag for a civ level entity (you can generate a world with a SLOW_LEARNER race, but trying to play fortress mode with them results in auto-collapse). I imagine the pet-forcing behavior of the token doesn't change the "no butchering sapient species" rule.
Not necessarily true. You can play with a slow learner race as long as you have at least one 'normal' civilian creature to buffer up the rest. (true with fortress mode too)
example:
[STUPID_AS_ROCKS_CIV]
[SITE_CONTROLLABLE]
[CREATURE: *Custom intelligent creature you've made yourself or a relevant imported animalman/other as a 'buffer' to stop worldgen crashes*]
[CREATURE: TROLL]
[CREATURE: OGRE]
[SUBSELECT_SYMBOL:WAR:NAME_WAR]
[SELECT_SYMBOL:UGLY]
[SUBSELECT_SYMBOL:PRIMITIVE]
[WEAPON/ITEM/ARMOUR/TOOL etc.]
*To stop inundation i would recommend a high starting pop and a relatively mid-level ending pop (5000 or so) if your citizens are going to be particuarly powerful (blind cave ogres for instance i toyed with) they won't spiral out of control or murder your lag with existing races (which preferably your race should smush quick)
[preferences/jobs/settlement sites & types + ethics: whatever you want or plan to use (i applied artwork values to my mixed race blind ogre civ once as a sort of in-joke/challenge on the basis that they can't see and therefore make terrible engravings and embellishments)]
[Positions: area to tread carefully around, this is a good reason for worldgen crashes, establishing site only roles is recommended and i would highly recommend trying to set your outpost liason to be your intelligent creature type (they can be elected if your races have a low lifespan to cycle them out, i have elected ones on my goblin playthough so im not stuck with the same person) avoid mayoral roles and druidic roles because these incur crashes and keeping broker roles the same stops issues with the trade depot]
The game will collect these and scatter the settlements with single race in each site (so ogre towns and troll towns but no typically mixed settlements), but because trolls and ogre's don't speak, everyone will be hostile to you (embarks will be random chance to spawn with ogres or trolls as citizens, rerolling for your intelligent race or having intelligent common domestics to suppliment is advised - for the purposes of roleplay i suggest hobgoblins or some kind of typically servant race,
ensure they have no [PET] tags but carry [COMMON DOMESTIC], squeaky voices are optional and you may opt for strange moodage or other tags just to make things interesting [prone to rage/alcohol dependent etc] this will also let you take them with you en-masse for a slave-force at 1 point each (unless you modify pet-value)
EDIT - Scratch that bit. Im thinking of the time i had my raws jumbled up and it enabled animal men citizens on embark. Though that was sweet, the incurring strangeness of everything else was not.