Missing pre-created families

@DJGeoRobThe culling of NPC- households (families) it most definitely related to your gameplay.
But to be sure: the maximum of Sims in the game will be 800. When you state that you already play with 313 Sims in the total of the game there will be some glitching and strange effects in the families; Yours and the NPC's. The game core will try to restore a certain balance in player and non player controlled Sims to make a game with a storyline possible.
However, probably the ghosts will disappear much quicker than the living Sims. And of course it will also depend on the amount of worlds (EP's) you own.
The use of some mods will only prolong the inevitable. And this type of gameplay will also put a strain on your system and RAM memory.

Anyhow.. Have fun playing. 😉

@DJGeoRob 

Ok, one of the good things is you can get those sims you listed back if you want to.  Just open a new save and go to CAS and save them to your library.  Then just add them to your main save from your library.  They won't know anyone outside of their household but at you'll have them and it shouldn't be much of a problem to have your sims meet them again.  

When the game culls sims, it culls the entire household.  It doesn't cull a sim in a household and leave other members of the household.  Since only Katrina disappeared out of the Caliente household but not the others, she probably died and you didn't notice.  My guess is that you have aging on for all sims so that would happen.  In a new game, she is an adult but the others are young adults.  The rest were probably culled since the entire household disappeared.  But if you played your story line long enough, some of them could have died off too.

Now how to have your unplayed sims not disappear again?  You should move all of the sims that you don't want to disappear in the My Sims side of Manage Households (left tab).  This will usually prevent them from being culled.  The game will always cull sims in the other households first.  To further protect them, make sure they are on a lot and show as played households.  Easy to do.  All you have to do is play them for a few seconds. Playable sims are never culled as far as I can tell.  You can tell if they are played by the darker color of the household in Manage Households.  A lighter color means they have not been played.

Now you have to consider if you want them to age and die.  So go to Gameplay options and select what you want.  There are 3 choices.  I forget the exact wording but it boils down to active household and all played sims.  So if you want those sims you lost and put back in your game as they are and not ever disappear by dying of old age, then turn aging off for them.

Hope this helps.