I expect nothing less from you Rysith, thanks for the Orcs. Can you elaborate on what makes a creature able to procreate?
No problem =)
Creatures can breed as long as they have a [CHILD:#] (and optionally a [BABY:#]) tag in there somewhere. The # is the age at which they move on to the next stage, so [CHILD:12][BABY:1] would make them be babies for a year, then a child to 12, and then adult thereafter. To my knowledge, though, it only loosely affects things outside of history gen (so non-breeding non-aging non-megabeasts won't go extinct faster than breeding non-megabeasts, for example). In general, anything that can civ should be able to breed (and thus expand), and anything that isn't immortal should be able to breed. Org's spiders, for example, would stop existing after the first 300 years of worldgen.
Mount-wise, I'll have to recommend the Orc's Armored War Elephants, which I've been lead to believe will reliably trash fortresses.
To get things to spawn with adamantine weapons and armor is a bit difficult. I believe, though, that if you remove [DEEP] from adamantine, remove [METAL_PREF] from the dwarves, add [METAL_PREF] to your enemies, and add some biome tags to adamantine (small clusters or single tiles in igneous intrusive, maybe?), it should work. You need to remove [METAL_PREF] from the dwarves to prevent them making everything out of adamantine, and it might take away your ability to make steel. You'd also need to make a new [DEEP] material, or the HFS won't have anywhere to live. Up to you what that material is, and it could even be something generally useless.
One more suggestion: multiply the size of all non-domestic animals by 10, double their cluster numbers, then embark in an undead biome, since undead animals are always aggressive without the spawn limit. Zombie groundhogs will be as large as whales. Zombie sturgeon will blot out the sun. Skeletal Elk will fill your nightmares.