On castes: you can have as much types of creature living together as you want, both male and female... But it will be considered a SINGLE creature of different variety. So you can have dwarves, humans and werewolves living together, but two dwarves may give a birth to a werewolf, or a human and a werewolf may give birth to a dwarf, because they are considered the same creature.
Although I don't see a problem of humans, vampires and werewolves being the same creature. Humans can become vampires and werewolves, right?
Arg, that's what I expected. I think it'll be fine enough, though - like you said, I'm willing to suspend my disbelief and say the couples are adopting.
I worry more about trolls marrying werewolves and other such monstrosities. Or perhaps if they have sufficiently different personalities in the raws they would not romance? Worth testing.
And you're partly right about humans, vamps and weres. In Discworld, vampires can both be born and "turned" by other vampires (I'm not sure if not only by the born vampires). And about werewolves, they breed with humans fine. Read up in the wiki, they explain better than I can.
But I think the graphics will be the easiest thing to come by.
But good graphics is something hard to make . I spend a lot of time finding shapes and trying different images to make something to look at least decent.
Human/vampire (I tried to make them smaller in comparison to the troll):
The elves are more Sealed Evil In A Can than a constant threat
Well, what about high triggers, so you get them only when you are very wealthy (so they start to be envious of your city being better than their).
Awesome! But hey, the 7 inch tall Disc gnomes should be a few pixels big then.
Elves are living in another dimension, together with their fanciful livestock such as unicorns. They are alien-minded, cruel, and treat other beings like a little boy who burns ants alive using a magnifying glass. They make vanilla gobbos look like care bears in comparison.
Oh, I got an idea - how about they are the HFS?
It's really the only city that does that, so I guess it's thematic if you only build one fort in a region. I'd rather have a choice of building a Dwarf or Werewolf or Troll only fort.
That's not an serious issue. You can always switch around the CIV_CONTROLLABLE tag, can you not?
Deon - are you able to make golems work? They are a pretty important part of Discworld, I suppose, and having them as a really expensive CAN_LEARN and CAN_SPEAK pet would be great. In such case, could one assign labors to them?