The only way to affect invasions is the TRIGGERS in the entity file. Of course some more prolific and successful entities will siege more, but they also kill more other civs in worldgen, making it a double-edged sword.
Best option in future will be the spawnunit version expwnent is working on atm, it allows spawning groups of enemies at the map edge with specific equipment, but it would mean "hardcoded" invasions, each one set up by hand by a modder.
Balance with only the in-built triggers is going to be tricky. I might need to rethink what civs I even include and how they scale. I think the problem is that worldgen can easily give the player a world where sieges are highly unlikely, leading to highly variable levels of conflict. I don't think it's an inherent flaw of SalmonGod's version, but a problem that Toady needs to fix in the next update with entity hostility.
I'm thinking I'll merge Stranglers and Beakwolves or remove one of them. They will arrive concurrently with Blendecs, Frogmen and Tigermen at the earliest trigger. The second trigger will see the arrival of nagas, ferric elves, pandashi, badgermen, and warwolves. The third and final trigger will bring elephants and hellfire imps (also nightwings). If I update the bonus races, I'll restructure them accordingly.
Early races may be likely to fight each other, but later races will use the same _THIEF tags so that they ally instead. EDIT: upon further investigation this does not guarantee they will not attempt to kill each other.
I think I will incorporate some of the bonus races into the base mod to make up for the lower chances of civilizations a) spawning armies and b) landing them on top of your fort. Judging from mods like MW, Dwarf Fortress can handle the high number of civs anyway. I may need to bump up reproduction.
Edit: Found an intriguing suggestion - make all races like Mountainhomes, and only tolerate each other's dwelling places, to focus the sieges on dwarves.
Going to make cave harpies that are amazing climbers. They will come in the first wave. Hopefully giving things good climb won't ruin fps.
Going to make Jotuns part of the base mod, and give them amazing climbing and jumping ability. Minotaurs will be superfast and great trackers.