Let me give you an example of the NPCs he is talking about. I call them Babble NPCs. They don't do anything but talk.
They aren't involved in anything. No missions. No quests. No events. No trade this for that. They do nothing other than give a line of dialog and that is it.
Here is a pull request I made adding babble NPCs to Bastok Markets [S]:
https://github.com/DarkstarProject/darkstar/pull/1252
These NPCs aren't involved in anything, they just exist. There are NPCs like this all over. Here is one NPC I added:
http://wiki.ffxiclopedia.org/wiki/Benjamin
Notice he is not linked to quests, missions, shops, or anything special in the wiki. So, you want an outpost in the middle of Bastok Markets [S], use him. He will not break anything. Put in your custom script and go.
You can find these "babble" NPCs in the locations you want an outpost. Put the custom script in and go.
If you want to know what NPCs are available, look at the wiki:
http://wiki.ffxiclopedia.org/wiki/Bastok_Markets_(S)
Here, I see many NPCs:
Adelbrecht (E-8) Campaign Enlistment
Adelheid Sturm (H-7) Campaign General
Adelinde (H-6)
Adjutant (J-9)
Those are just the start. There are many on that list.
Want to know if the NPC already has a script created in DSP? Look it up.
scripts -> zones -> your target zone -> npcs
You will see the scripted NPCs listed there.
For example, in Bastok Markets [S], I see these NPC scripts:
Adelbrecht.lua
Adelinde.lua
AngryBull.lua
Benjamin.lua
Blingbrix.lua
BottomlessGorge.lua
Christoph.lua
Clemens.lua
Heumila.lua
HomePoint#1.lua
Jagaris.lua
Karlotte.lua
Moogle.lua
Porter_Moogle.lua
RedCanyon.lua
Silke.lua
Weldon.lua
Another NPC that isn't there but is listed on the wiki? It hasn't been scripted yet.
Sorry, there is no master list. You will need to do your own homework, and look between DSP code, the wikis that whasf already pointed out to you, and try to figure out which NPC fits your needs.