I increased the each of the three different cave params in the world gen settings to 500, and I had *13* dragons and three demons, along with a few other creatures, survive in a standard world until 2050. That's less demons than some of my other worlds, but I've never had a dragon survive this long, let alone thirteen of the fell beings! Now it's time to brush up on history and start slaying some things!
Edit: I also changed the Dragon's info in creature_standard.txt to [SIZE:40] and [FREQUENCY:30]. Who knows if this did anything.
Edit2: One of the creature_standard.txt changes that I made must have severely increased the Dragon's survivability, which was a great success. There were no dragons killed before the year 300, while nearly all other megabeasts were dying in droves. Here's the entry I used, although the FLIER body part doesn't work:
[CREATURE:DRAGON]
[NAME:dragon:dragons:draconic]
[TILE:'D'][COLOR:2:0:0]
[MODVALUE:50]
[DRAGONFIREBREATH][FIREIMMUNE_SUPER]
[FANCIFUL]
[MEGABEAST]
[GENPOWER:10]
[FREQUENCY:30]
[PETVALUE:10000]
[GRASSTRAMPLE:50]
[BONECARN][PET_EXOTIC]
[BUILDINGDESTROYER:2]
[NOFEAR]
[STOUT]
[EXTRAVISION]
[liKES_FIGHTING]
[CAN_LEARN][CAN_SPEAK]
[FLIER]
[PREFSTRING:terrible majesty]
[BODY:QUADRUPED:FLIER:TAIL:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:MOUTH]
[BODYGLOSS:CLAW_FOOT]
[SIZE:40]
[ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:1:6:GORE][ATTACKFLAG_CANLATCH]
[FAT:10]
[ALL_ACTIVE]
[BIOME:ANY_LAND]
[STANDARD_FLESH]
[SWIMS_INNATE][SWIM_SPEED:2500]
[HOMEOTHERM:10040]
[SPHERE:FIRE]
[SPHERE:WEALTH]
[SPHERE:EVIL]
[SPHERE:MAGIC]
[SPHERE:DEATH]
[SPHERE:DARKNESS]
[SPHERE:SKY]
[SPHERE:LIGHT]
[CURIOUSBEAST_ITEM]