No kingdoms really know for sure where they are going to be yet... I'm a count and planning on Brudvir but if my kingdom winds up somewhere else? Well I will be going with them of course.
If you are set on your tribe you might need to wait and see how domain selection plays out before declaring, I don't think you're in danger of running out of counties to join anytime soon.
You may however have an issue with 'adjacent baronies' as baronies are specific to mayors who have a contract with the duke to establish defenses and train soldiers and there is not much reason to have 2 barons very close to each other (or even in the same county). Without a defense contract from the duke a baron is just a mayor, though they may be a mayor with a defensive or military styled town if they want/can afford it.