So dwelling on this more, I think it’s fine if we don’t have any inherent mass-related impact on tolerance ranges. Since some parts will have an inherent effect on tolerance ranges, more complex organisms will naturally progress towards specific ranges.
I think it would be nice to have certain parts, like nitrogen-fixing plastids and hydrogenase, work negatively with oxygen, necessitating more adaptations or actually incentivizing certain playstyles to seek anaerobic conditions as long as possible. So if that isn’t too difficult of a mechanic to implement, it could be a nice way to introduce a level of depth.
As to mucocyst, that could be interesting in relation to sunlight, as encasing yourself would protect yourself from inadequate conditions. However, beyond that since things like temperature and such don’t vary in a single life, I think such a thing would best be represented through auto-evo if implemented.