Hello there!
Guys, I'm sorry for no updates during these 2 years. A lot of things happened but now I can say that I'm back on track)
So. As ebmar suggested I made g_a_clothesXX files and added them to certain npcs
I tried to change this way the outfit of the first officer on Taris (g_sithtrooper002.utc). But as I understand G in the filename means Global and this file is used both for this npc and for sith soldiers in the battle suit (at least the same npc is used on the Endar Spire, 4 sith soldiers in the last room before the escape pod with Carth ). As a result, if i change the battle suit to my g_a_clothesXX for this npc it will break the game (these 4 soldiers will change their appearance and become frozen. They can't be killed and script that let's you open the door afterwards won't fire). This was the original blocker and one of the reasons why I stopped working on this mod.
The other reason was that on Manaan before the Sith base (manm26XX modules) the only npc that changed appearance was man26_sitharg.utc (sith who is arguing with republic soldier in the first hall after the docking platform where the Ebon Hawk landed) Others didn't want to change though I added g_a_clothesXX to their inventory and textures were for the same models as these npc used.
For some reason it didn't work. Even for unique NPCs like Viglo, Duan and Vek. First two are drunk siths in the bar and the last one is the Twi'lek in the sith uniform. (The other thing is that I use K1RP mod. There's a little chance that maybe this is the reason but as far as I know this mod doesn't modify these npcs)
But evetually I found out that in the modules where players is fighting with sith it IS WORKING. I mean, it works like a charm.
So I've changed sith npx on the Taris sith base and on Manaan sith base
tar09_recepti001.utc (the poor Twi'lek girl on the reception. I gave her sith battle suit whick was made by redrob41 https://deadlystream.com/files/file/726-modders-resource-specialized-combat-suits-for-kotor-and-tsl/ ) - actually i don't know if I can add this to my mod but this is a work-around because I didn't want to edit appearance.2da and I didn't want to change model for this NPC. And in the end I like the result better than the original idea (You know, she isn't a consripted soldier, she just works there)
g_sithcomm003.utc (sith Captain. Though its a global file this one worked). I checked only Taris and Manaan, if this file will break something like g_sithtrooper002.utc I will sadly delete it. But for now everything works just fine.
n_sithcomf002.utc (I've changed several techs to Sergeants. They looked to similar)
tar09_shieldtech.utc (this tech is now female with rank of Sergeant-Major)
tar09_sith092.utc (common sith trooper in the battle suit. Changed it to soldier in the Trooper rank without armor. The idea is that no one is waiting for the ambush so some soldiers are without battle suits)
Ow. And one more thing. I added slightly more HP, medpacks and shields for these npcs (except Senior Technician) because without their original armor they're weaker so this way I balanced it a bit.
Moreover, I left other techs in the original uniform and aside from tar09_sith092.utc there're other sith soldiers in the battle suit. So this doesn't ruining the original look of the npcs at the base. Just gives it variety.