So glad to find this mod; I had a similar idea for this a long time ago i put up on the forums, so I'll put up what I have as suggestions that could be implemented. The idea I had would be that different types of life would have traits that would predispose them towards certain ethic types, albeit not being locked into them, or have traits that favor certain playstyles.

-Fungoids consume minerals as well as food, splitting their feed options between the two. Additionally, they receive a percentage-based modifier for their growth rate based on the proportion of non-fungoid pops living on that world, since in nature fungus' role is most commonly the consumption of dead biomass, effectively giving fungoid immigrants a significant pop growth bonus. This also benefits multicultural societies, making them lean towards xenophile society models for peaceful playthroughs, or effective conquers forcing themselves on existing societies. They would receive a penalty to growth on dry worlds, as they don't do well in direct sunlight and dry environments.

-Current benefits in mod for Arthropods are fitting, although I would say instead of a habitability bonus they receive a large housing space reduction. Their pops should have a low society sprawl impact.

-Humanoids being the politically-focused aliens is an ideal route, with leaders getting a default bonus to statecraft research rather than their pops generating extra society research. I think giving them a reduction to society sprawl would be a bit much, so keeping them as a "midrange" species without specific modifiers and a lower reproduction rate would be good.

-Mammals I think could benefit from having an extra trait point. In nature, Mammals are a dominant form of life on earth due to their ability generate their own internal body heat, and can adapt their diets and behaviors better to sudden change better than any other form of life, including arthropods. They would however consume more food than others, reflecting their much higher metabolism.

-Reptillian bonus ideas are on-point here, no change needed.

-Most avian species can fly, and even those that cannot are very close evolutionarily to those that can, akin to bats and mice being more dissimilar than seagulls and penguins. This in addition to being warm-blooded like mammals should gain them a decent habitability bonus (10%) for their inherent mobility and resilience, as well as a 10% bonus to emigration, stacking with Nomadic. They would get a penalty to army health, from having hollow bones.

-Molluscoids, being primarily water-dwellers, should have some unique habitation mechanics and traits associated with them. An "aquatic" trait should come as optional for other species but default for molluscoids. This should add special building for them called "Aquatic dwellings" which elliminate molluscoid housing usage up to 10 (more for upgraded versions), similar to how organic sanctuaries eliminate housing usage of bio-trophies. Coding for it would like kind of like
If pop molluscoid: enable "Aquatic Dwelling" building
If "Aquatic Dwelling: on planet = yes, reduce Molluscoid housing usage to 0.

-Plantoids would consume half food and half energy, akin to Fungoids using half minerals. In addition to this, they would receive a large (20%) bonus to food generations, but gain a -25% bonus to pop emigration for their sedentary nature.

Let me know what you think in the comments!