I've done a lot of civ-modding, I think I can help with some of your questions.
You don't actually need both [CAN_SPEAK] and [CAN_LEARN], [INTELLIGENT] does both.
Being genderless will prevent reproduction, you need both males and females. You can make the genderless caste far more common then the gendered ones with POP_RATIO tags.
You don't actually need any particular organ to speak, and language doesn't matter except for what the civ names things. The exception is the [UTTERANCES] tag kobolds use. That isn't a real language.
I don't think the GCS's will kill all the other civs if you don't give them armor or weapons.
Simply having clothes specified in their civ entry will produce clothes so large that other races can't wear them, I think. Anyway, people won't wear other culture's clothes unless their cultures merge somehow. For example, a spider bandit leader or necromancer will cause his minions to dress like him even if they aren't spiders.
Letting them grasp things will need minor changes to their body. You'd either want a custom bodypart or to add [GRASP] to one of the ones they already use.
You also need [CANOPENDOORS] or they will be unable to open basic doors.