I want to make Gabbro men, Blizzard men, Molemarians etc into their own non-playable goblin-like entities. Do I need to add [INTELLIGENT] to each creature or will they work out of the box?
If you want them to actually do anything other than terrorize their neighbours, yes.
If you just want them to mindlessly go to war with everyone, then no. In fact, not being able to speak helps matters, as they would not be able to negotiate peace, leaving everyone else hostile with them by default.
Okay, it seems to work so far. I started out with Gabbro Men, Amethyst Men, Blizzard Men, Fire Men, Ogres, Manera, Molemarian, Plump Helmet Man, Satyrs, Sasquatch and Gremlin civs enabled. In the first worldgen, it looked like most of them got destroyed early on by megabeasts. Only Ogres and normal civs showed up as neighbors on the embark screen. A short fortress mode was visited by ogre and yeti zombies. Since yetis shouldn't have been one of the available civs, I wonder if they became someone's minions. In the second world, I turned down megabeasts to the lowest, and it was the same story, only Ogres survived. This time I tried it out in adventure mode and found an empty Dark Pits that once belonged to Fire Men. Legends mode suggests that at least one Fire Man civ was destroyed by ogres. The first two worldgens both struggled when placing civs so this time I removed gremlins from the list, assuming that if anyone was going to cause trouble it would be gremlins, and behold, no problem placing civs and the third world shows Gabbromen, Blizzardmen, Molemarians, Sasquatches, Satyrs and Firemen. No Ogre civs show up in the history, it's possible that ogre civs just cancel out all other Dark Fort civs if they exist.
I'm curious, are there any obvious red flags that make a Gremlin civ impossible? Would a kobold-clone gremlin civ work better than a goblin-clone gremlin civ?