you can literally make the ratios for females 1 and males 10 or 100 or whatever.
And you CAN make them breed like animals, by making females non-intelligent pets, and just letting them mingle with males. They should get pregnant from any males available even if the males are intelligent, but if that doesnt work, you can make an equally small portion of males unintelligent. This worked for ant people civs I've played around with before.
children will have the (tame) string after their names, and might not list/know their parents properly, but otherwise should behave normally.
Oh, and the game might try to allow unintelligent members in noble positions. There was a discussion in the creature + entities thread about that. positions filled in worldgen or world simulation during play dont always seem to check the individual they've chosen is actually elligible for the position, so you might find an unintelligent member of the species in important positions in NPC-controlled sites. You can get around this with the allowed_caste tags, but electable positions and positions that are auto-filled like expedition leader will still allow unintelligent targets. Rarity of these individuals also helps.
Your expedition leader position could have no valuable responsibilities besides appointing another, higher position which then appoints commanders, bookkeepers and meets with workers etc. The expedition leader might get filled in with an idiot, but it shouldnt matter, then.