I think having to both take an ethos, and pay 2 trait points would be sufficient to allow someone 'innate' ethos.
Example, a highly aggressive species with a trait "naturally aggressive" or something akin to that, which locks them at militarist, they might go down from fanatically militaristic, but their pops will never become pacifists or neutral in regards to militancy, they can still become collectivists, individualists, spiritualists etc, but they just won't drift on militarism, it's ingrained in them.
You could limit it such that no race can have 2 of these traits, just one, and all it would do is allow races like insects to be collectivists by nature, or on the reverse side a race of birds or something to be naturally individualists etc.
I don't see how this would be unbalanced, their giving up significant advantages to simply not deal with ethos divergence on a single axis.
To put it in prespective conformist (2 points) reduces all ethos divergence by 20% this would for 2 points reduce ONE ethos axis by 100% (or rather, prevent it from sliding entirely out of the ethos) that's essentially equivelent to conformist, just focused in on a single axis of the ethos 'wheel'.
Also lets look at a recent scifi example of innate ethos.
Hunters in Halo, they are Innately fanatic collectivist hiveminds, being that each 'individual' is composed of thousands of worm like creatures, and then further divided into groups of these individuals that operate as hives. The Zerg from starcraft, are an innately (modified to be so) collectivist hive mind, whereas the native zerg are an innately individualist group (with power being what determines strength, and individual advancement being key)
Mass Effect, Asari, They are naturally xenophilic due to their bonding methodology and culture, and practically nothing will move them from this.
The protoss from starcraft, as the Kala (their psionic link) enforces their collectivism, with the only way to end their collectivism being through a Massive upheavel (or rejecting joining it in the first place as with the dark protoss) and only ended it in response to losing their homeworld (and having it corrupted via some space magic) so in other words only via an event.
Also pretty much every X-com alien race has genetic predispositions toward specific behaviors.
All of these are good examples from post 2000.
If we go back further than 2000 (but certainly past 1940) we get the entirety of startrek, warhammer 40k, and even many races in starwars all having racially determined personality traits.
All I'm saying is that there is definitely modern examples of racial/genetic inclinations in culture, behavior, and 'ethics'.
Also
https://www.boundless.com/psycholog...the-influence-of-genes-on-behavior-137-12672/
The effects of genetics on human behavior are Well documented and continue to be proven by study after study. Who is to say that an alien species couldn't have stronger or more prevelent genetic heritability for genes relating to behavior or specific philosophies.