Personally i reckon the Tentacle Demon was a better purchase. I've decided to let the GCS's roam wild and free and they seem to be amusing themselves by waiting on the left side of the map for the yearly snailmen, slugmen and animal migrations and eating them.
Hell i didn't even know you could tame Tentacle Demons.
Without modding, you can't. Of the other creatures you mentioned, you also need to mod the game to get tame Antmen, Fire Imps, Spirits of Fire and Magma Men.
By the sounds of it, you've modded your game so that every creature has either the [PET] or [PET_EXOTIC] token.
Yeah, i did that ages ago in like my 4th fort when i set up next to a Chasm that had literally 60 Antmen in it and was all like "Awesome! Butcherable Nonstandard! Tame em and eat em!" then discovered they weren't edible.
So i just ran through the raws and added [PET_EXOTIC] to all the animalmen as well as a bunch of other random critters, didn't think i'd added it to Demons though, although a cursory check of the raws shows that yes i did.
Also just modded Antmen to be war trainable, the last 3 Elven caravans each brought between 12 and 24 Antmen, 1 - 3 Tentacle Demons and random other creatures, including a second Spirit of Fire.
My two GCS's finally went down to a weighted mass of Orcs that promptly fell into my Tentacle Demon Pit and were eaten and i'm currently constructing an elaborate defence with the Spirits of Fire to set everything ablaze.
I'm also build an Antmen hill which i will populate with my 71 strong Antmen army, for the lols.
If you offer items of WAY too high a value in a trade (Trader Profit : $$$$$!) the caravan will bring more goods next time. So I'd spend these crafts to buy whatever the hell I wanted. Steel Pick? Sure! Why not. It does not work this way if you offer the items as a gift.
Yeah, found that out with the Human caravan came back with 40 pages worth of items. Ffffuuu-