The Virus Injections are a pretty good example of this. Say 20 mobs are supposed to spawn, each with one of 16 assorted personas between them and only 12 available slots (hypothetically) at any one time. Odds are at least a few of them will share a persona, or the player kills some of them before the 13th NPC spawns, and there's no issue. However, the first dozen NPCs may load in just fine, but on the off chance they all have unique personas, lucky 13 is going to have whatever is available in memory. Could be an old male bum with a young female voice.
As for work-arounds for persona assignment, I believe there's only overriding and supplementing a persona... but a supplemental persona is still just an override by any other name. Just a different approach.
I'm not sure there's a quick and easy way to bypass this issue. I've added this to my TDL for the weekend and hopefully there's something simple I've overlooked.