Civs can only domesticate the creatures they come in contact with, so keeping them subterranean will prevent Humans and Elves from finding them. As mentioned above, let the Dwarves use cave creatures and not the Goblins or Kobolds.
For the body, look through the body_default.txt for the bits you want to assemble, then look for creatures that use those parts. Since I went through all of that trying to build a similar-looking creature, I'll post what I ended up with:
[BODY:BODY_HEAD:4ARMS_STANCE:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:SPINE:BRAIN:SKULL:MOUTH:TONGUE:4FINGERS:GENERIC_TEETH_WITH_LARGE_EYE_TEETH:FACIAL_FEATURES]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[REMOVE_MATERIAL:SKIN]
[REMOVE_MATERIAL:HAIR]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[REMOVE_TISSUE:SKIN]
[REMOVE_TISSUE:HAIR]
[BODYGLOSS:CLAW_HAND]
In my case, I replaced the SKIN with a custom tissue that varied by caste. Similar-shaped vanilla creatures have ribs and throats, which I think is ridiculous. In your case, it probably makes more sense to drop the SKULL and use the exoskeleton materials and body detail plan (which removes the need to remove unneeded materials). The actual creature is in the mod in my signature, but the similarities to what you're doing really are just "skin deep."
If you want a head, thorax and abdomen, the easiest thing would be to copy the INSECT body plan, delete the second pair of legs and feet, rename the third things to second, and play with the relative sizes.