This might be a bit of a stretch, but this is my answer:
1 (One)
While I can be the answer to everything and everyone:
People can refer to themselves as "one". "I am just but one of many." "One is glad to be of service."
Too much of me and your demise will be spun:
A group of individuals (ones) can bring about a rebellion. "United we stand, Divided we fall."
I can make things strong:
This goes back to groups of individuals. A group of sticks is far stronger than an individual stick, or 2x4's, steel beams, bricks, or whatever individual building block you want to use.
But transition is where I belong:
Transition (change) is often started by an individual. One person can start a movement, one rock can start an avalanche, one animal can start a stampede, etc.
I am not natural, as is the common plant:
Individual items are not natural. Things may be unique, but they are still similar to something else. A single animal or plant either evolves more of the species to be like it through mating/crossbreeding or it becomes extinct. When we create a single atom that doesn't exist in nature, it degrades quickly. Plant a single seed and it sprouts, creating more seeds, creating more than one plant.
But in size I am yet but a paltry ant:
Compared to most numbers (million, billion, septillion, google, irrational numbers, etc.), 1 is tiny in quantitive measure or amount of digits.