I personally think that poison heal should be exclusive to snaelstrom, making it x -> x -> poison heal, despite it being water/bug during the second stage.
My reason revolves more around the nature of the second stage, which to me doesn't seem strong enough to digest poison, having just evolved from a small snail or a slug of sorts, it doesn't fit with me that it can digest poison and use it to heal itself in the process, that's something which snaelstrom to me feels like it fits, being a big snail with a large shell, having the ability to digest poison and use it to heal itself seems like something that requires alot of energy to do, as it would change in its dna regarding the ability to digest poison. That is not something I see the second stage being able to do quite yet, as it's not strong enough to be able to adapt to the ability to eat and digest poison for health, and are more I guess brute force if they were to go against toxapex, not for food.
For caribaby I'm more leaning towards x -> x -> galvanize.
Galvanizing is the stimulant of natural electric energy, or in more simpler terms, charging electricity to resemble that of natural electricity. This process is not something I see neither of the first 2 stages doing, as they simply do not have the size for it (When I imagine caribolt galvanizing, it could be turning energy gained by either the sun or by rapidly moving into electric current, something which requires a body structure that can handle it) and don't feel strong enough to do so.
Those are my reasons for having the hidden abilities of the fully evolved snaelstrom and caribolt be different from their pre-evolution forms :)
(Just kind of hoping I did my research correct and not looking like a fool spouting random things not related to the things I'm trying to convey, if so please let me know)