(side note: carp aren't vermin. that's why they can hurt you. I don't think you can make a vermin civ; and anyway a single cat could make it extinct, so it's probably not a good idea in any case)
So I think, Lyrax - and this is going off other things I've read on the board here - if you place a fort and it is within range of more than on EVIL or NUISANCE entity, it will pick one at random to make contact. If your fort is near goblins, and giants, and minotaurs, and each of those entities has ENTITY_GROUPING:EVIL, then you should only deal with one of those.
This is not from personal experience, however, so doing some testing and then editing the wiki with the results could be useful.
Download and unpack a fresh copy of the latest DF version, put it in something like DF_WORKING or some other folder where it's obvious it's a test-bed install. And then do the following to /raw/objects/entity_default.txt :
* Cut and paste a copy of the goblin entry to the very bottom of the file, below the kobolds
* change the CREATURE token to whatever you want (minotaur, giant, etc)
* remove all of the WEAPON, ARMOR, et cetera tokens at the beginning of the entry -- entities don't need any of them to spawn, and it will slim the amount of text you're looking at down to a managable level
* set their START_BIOME to ANY_WETLAND -- this should stop them from taking over land that would otherwise be occupied, and give them a better chance of making it to 1050.
* give them the token ADVENTUER_TIER:4 -- it can be anywhere in the entry, just make sure it's enclosed in square brackets like the others. This will let you start them as an adventurer, which can help to see where their settlements are (and see if they're making them at all)
* save the file
Now, generate a world and see what happens. Make sure to check errorlog.txt in the base directory (it will often generate as soon as you start the game if something is wrong) for issues. If the world generates fine, check them out in Adventure and see if they have settlements. If so, build a fort near one and play for a couple of game-years to see what happens. I'd suggest not making too many defenses, since you want to entice them to come up and interact. There should be something on the current page of posts, or maybe the next, about progress tokens.. I think you can change those to make them show up sooner, rather than having to wait til the second year or more.