Of course it depends on how you're going to implement it, but since I assume that your Thai civ will have different settler maps, city names etc. you have to do it similar to me which meant that I added another dimension of size 2 (respawned/unrespawned) to all maps, UP and UHV text files. That made sense back then because:Well, i neither looked at your Italy/Safavid code nor Jarkov's guide how to add a civilization, but is it really that much harder?
1) I thought adding another civ was more work than it actually is
2) I planned to have the system work easily for respawns of other civs
But it's really not worth the effort for only one civ. Most of the work of adding new civs is even filling in all the various modifiers, and that's easier to do for you as well since it's exposed to Python.