I say that a multiracial fort would get people out of the monotony of an average fort. Citizens with a body size larger than, say, 75,000 could have bad thoughts like "Worked/Slept in a far too cramped tunnel" unless the underground had an extra Z level. Flying citizens could have bad thoughts like "Was unable to stretch his/her wings", and good thoughts like "Had a good/great/fantastic flight recently". Amphibious citizens could have bad thoughts like "Was unable to/Did not have the time to swim", or, "Swam in dingy/filthy/absolutely abhorrent water lately", and good thoughts like "Swam in uncontaminated/good/wonderful/comfortably heated water recently." This could add incentive to add human-like villages to the larger, surface civilians, aviary-like villages for the flying civilians with tiered housing and shops, and communes with water roads and heated pools for the amphibians.
Why would you go through all this work, though? Ask the poor kobold thief getting his nose punched though his spinal column by a pissed-off Tigerman Champion. I'd imagine that clothier's shops/smithies/bowyer's shoppes could have extra options like "Make Tunic (Bark Scorpion Man Sized)", or "Forge Longsword (Parakeet Man Sized)", or even "Make Crossbow(Grasshopper Man Sized)". The size modifier could be operable by some unused keys, and could result in a fort with every race occupying it happily clothed and armed, like a little multi-species utopia.
And if the Animal-Men are fleshed out further (and useless ones weeded out), we could have varying attributes to help each individual find a role for itself in the fort. If Mantismen got far higher fearlessness averages, A particularly hardened Mantiswoman with a knack for siege engines would operate a ballista, stand her ground and fire missiles right until the enemy is occupying the same tile as her. If turtlemen and other 'slow' races of creatures had higher tolerance averages, a Pond Turtle Man would make a great mayor, as his higher averages would allow him to easily grasp Consoler and Pacifier skills (and his office would probably have a jacuzzi). Of course, this is just averages, as there would still be hot-headed Turtlemen, skittish Mantismen, and lax Badgermen, etc, just as there are nice goblins in human towns.
An added thing: if you somehow, against all odds, nab a friendly Giant or other intelligent megabeast, or even a Titan or particularly well-mannered Forgotten Beast, they could have ridiculous living condition needs. The "shallow ceilings" thought could be amplified tenfold, and thoughts such as "Was put off/depressed/absolutely enraged by a lack of wealth as of late". Maybe they could even demand sacrifice, depending on their personality (a friendly, social Ettin/Cyclops/Giant wouldn't need sacrifice or wealth, but a mean, domineering one would throw rages when they don't get a sacrifice in their name.) This would tie in to how civs would worship powerful beasts, and I think it would be awesome to first-hand overseee a Mountainhome with an in-house Megabeast king or something.
Sorry if this is a tad off-topic, but I wanted to bring up some points without touching that "slavery" territory.