Along with Invite Over, there seems to be several other functions of MC where a sim (or at least this sim? -- the world only has one auto-generated one) in the mermaid household doesn't appear as an option.

Is this intentional? I know mermaids are supposed to be somewhat special, but I can use MC to add it as an occult state.

It's not the occult state that is causing the mermaid to not show up, mermaids can be residents too. it's probably that they would be a Service Sim. Can you fiddle with the MC filters a bit to get homeless NPC Service Sims to show up?

OK, I have determined that the mermaid household is categorized as a service sim.

However, it appears that I can only "Invite Over" a small subset of service sims -- I presume the ones that are currently in the neighborhood.

If I use MC->Status, I get the choice of, well, more service sims than I think I should probably have in this neighborhood (I don't use Manage Homeless because I've found that to be too aggressive in deleting homeless). However if I select MC->Invite Over, I can only choose from 17 service sims.

Yes for whatever reason Twallan blocked that on service Sims not currently in the world. Possibly because it's not really a good idea to be messing with them because the game will waste no time to spawn more and more if the current one's are already in the world when needed.

OK. I guess I'll have to move him into town.

Tangentially, I do appear to have way too many service sims. I'll take a wild guess that I ran Service Sims Out on the Town sometime in the past before realizing that was a bad idea. Do unused service sims have much effect on performance?

Oh, wait, I don't think there's anything wrong with my homeless/service population, at least the human one. I have a LOT of pets classified as service sims. I assume strays are classified as service? I guess that makes technical sense. I may need fewer strays.