If you are still figuring out the Ant civ breeding I have something that will work for you:
4 Main Castes
Queen [Female][Pet_Exotic] --This *should* make it so you can't bring it at embark/order from traders
Worker_Female [Female]
Worker_Male [Male]
Larva[No Gender][Pet] -- you want to be able to buy this
+sub castes for soldiers or w/e you want for castes better at fighting, size, what have you
Queen and Larva have a POP_RATIO:1 and the other Castes will have ratio choice multiplied by 10,000
Worker_Female POP_Ratio 20 * 10,000 = 200,000
Worker_Male POP_Ratio 30 * 10,000 = 300,000
This is so natural births won't produce any Queens or Larva[caste]
But wait they can all breed how can I get the civ to reproduce during worldgen? and not in game? Simple:
You create a interaction, this interaction gives [STERILE] (only way to attach that tag is via syndrome)
This interaction is on all normal Female castes and only target themselves. Worldgen these interactions are not used thus can breed until they are loaded in adv.mode fortress mode and use it on themselves, making them able to marry but not reproduce.
Now why Queens as a Pet you cannot buy, but Larva.
Meph (and my own outdated mod) use workshops to more or less convert creatures to other creatures. You can buy/embark with some larva, use a worker (feed larva royal gel (as far as I know this is what bees do when picking a new queen??)) worker can turn 1 larva into a queen ant caste. Now since both are pets they don't need marriage to reproduce, and since normal female are sterile but normal males are not (they don't give birth) they will magically breed with the queen, having kids. And since the pop ratio for Queen and Larva are so rare they may never happen, the queen will give birth to workers, and other castes that you ranked higher by multiplying by 10,000.
From there you can have things like if there is two queens they will both go berserk/insane (or just one) by setting the other ones DROWSINESS to a really high number (what trapped traders use to go crazy) so you can make it seem there can only be one queen in a fort. If you can still bring QUEEN castes remove the pet tags from it, since you can't remove the PET tag I believe it transfers so the Larva having the PET should suffice. And since the larva is genderless you can pick what type of caste you want them to be (Maybe mindless warriors that can be made to selflessly defend the meeting hall) up to you. Hope this helps your Unique Civ style