Goblins are probably the easiest ones to mod because they are 0 upkeep and malleable to what the player raw edits, i have my own 'build' of a goblin centric DF edition i've talked about before that's balanced because turtling is punished by inevitable infighting. Alternative races are either so far out there or not really on the planned dev plan at all that its not worth worrying about too much and instead assimilate the new dwarf features in the alternative context.
Eh. That's really cheating though, goblins are kinda broken as-is due to not needing food or water. Thanks to Toady forgetting that we now HAVE goblin adventurers in vanilla, and some tosspots seriously overthinking the moral implications of what was clearly a workaround to older instances of carnivore civs starving themselves out (nevermind kobolds no longer seem to do that).
I've done similar. All the non-playable civs need site positions defined at a bare minimum. Which that out of the way, humans are easiest to mod in. Goblins less so if you want trading, but they at least require less fuckery to retain a functional trade and exports system than kobolds, because only caged animals will fail to affect imports, instead of EVERYTHING YOU SELL.
Kobolds are next on the list, for the reason mentioned above, plus more work is required if you want a more primitive experience.
Lastly, elves are legit fucked because you need to work around the "no way to get grown wood" issue.