Will "natural enemies" (Babysnatcher civ and non-babysnatcher civ, etc) join alliances together if threatened, or do they stick to their own?
Item theif civs like kobolds are a seperate group also (tolerate each other with no hostility like babysnatchers), but since kobolds [UTTERANCES] it is not possible without some sort of oversight for them to act diplomatically, so that applies to modded races only.
Goblins accept tribute, so after a change of demon leadership and keeping up with artifact demands & again somehow improving the relationship standing of leaders mutually (in ways not added yet, might just hinge on them finding common ground) I think this is possible unless Toady explicitly says its not. Might fall apart very fast though on another leadership change or a diplomatic incident like forced tribute arrangements do.
Do distant in far away lands or normally hostile (see shonai's quoted green question on goblin allies) allies trade with your site or alter their behaviour after signing a agreement?Because beside poking them with a raid or summoning caravans with DFhack (as the masterwork mod does, though thats not applicable here) you'd normally require to embark closer to them. Goblins can't trade but not ruling out peddlers in the future, it could afford us some respite from babysnatchers while the alliance (/truce?) is up.
Right now im pretty sure the only way you can get goblin instruments (cave croc bone drum has pride of place in my tavern) and miscellaneous things is by exorted tribute or looting.