In an ideal worldgen with all 5 types, # of civ, the # the player sets should be divisible by 5. The civs will always spread out 1 hu, 1 go, 1dw, 1 ko, 1 ef, and then 10, for 2,2,2,2,2. When less conditions like vjek mentioned are met, I suppose divisible by 4, 3, 2 etc. should be picked.
Kobolds req Mtn or Nonmtn caves. You'll wanna null caves to get rid of Kobolds. Not sure how this effects outlaw dwarfs and crime/justice, as they setup shop in those places, too.
The problem with dorf only worlds is then you lose the foreign food, armor & wpn features. No daggers, no blowguns, no masks, no tree drama and exotic pet trade, no human seafood and longshank monster slayers, etc. Foreign items, like, facemasks, thongs/loincloths(legs and feet are optional), and high molar mass whips are, or were very powerful and game breaking. Dunno, now. This may no longer be true, as of musical instrument patch, which seemed to shuffle foreign and normal recipes about some.
Very long histories cease to be relavent after 1000yrs of titans, for me. After titans are gone, world historic figures blossom from those 5 types. Also you'll get lots more multi 'named' mega, semimegabeasts, and savage(giant variants). Dont overlook that giant fly, he is a giant's Giant. Ever pull up his raws? Holy cow.
Which, lotsa semi's is something the hardcore(no cheaters)seem to enjoy, in their worlgens, as semimegabeasts, unlike their big cuz fullmegas and titans, can be trapped and bread in captivity for truly awful things. Like a Rancor pen, in Star Wars, but with Rancor breeding. Imagine your own maze/moat, with a family of high strength and disease retardent Minotaurs to unleash on the an uninvited guest, pathing in from embark edge. When they turn back on you, there is your field of metal traps to protect you.