I'll admit this was all inspired from recently learning about the life cycle of jellyfish and their two life stages. The purpose for all living things is to reproduce afterall.

I feel nerdy that I become fascinated in this stuff but do not wish to become a scientist. I prefer science fiction.

--------

The Onion's ability to reproduce a single pikmin seed after an extinction is possible probably from a vacuole that stores emergency nutrients like in plants and can release it like an animal's liver if starved of food or to produce a pikmin seed.

Pikmin must be closely related to carnivorous plants as they respire like animals for chemical energy -hence they can drown quickly. They would use this energy as a metabolism.
Pikmin would still need to do photosynthesis to be able to respire but they would use respiration more since their body is less efficient for photosynthesis. If pikmin were any larger than they are now they wouldn't live for very long.
Drinking nectar helps speed growth dramatically and it has so much sugar that the pikmin probably needs to expend it, causing it to flower instantly. They probably use nectar as an extra food source because pikmin have an irresistable instinct to pull special grass where nectar can be found.

They are closey related Creeping Chrysanthemum because of the ambulatory root structures, but they require no photosynthesis and instead prey on creatures. This allows them to be larger.

Now to explain how they all have special elements and abilities... I'm joking! xD

This is all a fictional pikmin theory in general made by me. Unless it can be argued. Than that would be a prediction.