Maybe adding the [CRAZED] token in the creature_standard.txt entry for the gobs specie can help?
In the earlier versions it did not affect worldgen in any way, unfortunately. Most probably still doesn't.
Goblins take away people as slaves when they successfully raid a site that get converted into citizens because they're dragged back to settlements to work, so turning their slavery off in raws can be a easy fix to that issue by giving a large or slighted negative (like MISGUIDED which is -1 below the justifying it under special reasoning) to the ethic raws.
The OP's problem, if I understand correctly, is not so much about the goblins having other races in their civs, but rather other races' civs having too much goblins in them, due to the modded goblin fertility. Tinkering with the slavery ethic probably won't make any difference on the aforementioned issue.
However, dashadowlord can try and remove all the LIKES_SITE and TOLERATES_SITE tags from the goblin entity and then add back the [LIKES_SITE:DARK_FORTRESS] and [TOLERATES_SITE:DARK_FORTRESS]. This should minimize the goblin drift to other creatures' sites, while encouraging them to go back to their native sites whenever possible. Increasing the MAX_SITE_POP_NUMBER helps too.
Do note, that any of these changes may cause a huge population boom for the gobs, and walking into their sites in Adventure Mode will produce unbearable lag. On the other hand, it seems that exremely large number of goblins is what dashadowlord is after to begin with.