KittyTac, I'd argue that GC's realism applies only to plants, and that cavern critters couldn't maintain the size/fireyness/numbers they do now with such a low oxygen supply. Either you have (relatively) realistic flora and make cavern critters [NOBREATHE] and something to stop fire-composed creatures from just dying, or you could have (relatively) realistic fauna and have oxygen production sans-light, or caverns are going to need a total rework. Fire men in particular would likely be difficult to have in oxygen deprived caverns, given how deep you have to dig for them to appear.
GoblinCookie, firstly, default DF is fantastical. Even after the fantasticality slider is introduced, only the very bottom setting will be truly mundane. Not planning for the fantastical would be mindbogglingly short-sighted. Not to mention, that mechanic we're talking about? Bedroom visits? Works even when nothing fantastical is present, since a medieval subterranean fortress is pretty fantastical, and lighting/ventilating an above ground fortress is way less hassle, even in your scenario.
Secondly, multi-tile/level creatures will change architecture, not restrict it. Needing a wider/taller hallway isn't restricting, it just means you're gonna need a bigger boat. Only way you're gonna find it restricting is if a) if multi-tile/level doors aren't implemented at the same time, or b) if you have extravagantly large fortresses that take up the whole map both vertically and horizontally already. Besides, how often do you actually want big creatures inside your fortress? If they aren't grazers that should be in a pasture, they're usually trying to smash your dwarves into paste.
Finally, and this goes for everyone, this debate belongs in a thread of it's own. The subject of the debate is now completely unrelated to the subject of the thread, any vague relations to the thread have faded into obscurity. If someone wants to make a thread for this and link here that's fine, but this thread is about dwarven social interactions, not lighting and oxygen, or architectural freedom vs botanical realism