Is scalable growth the main reason why Darwinian systems select Fibonacci sequences?
I really wanted to put my thoughts into words as to why biological systems conform to Fibonacci's series.

What you say is entirely plausible and it does make sense. But what I wanted to address was why do these systems conform to it.

It is because of the interactions with the environment

Explaining this concept on multicellular organism first requires answering the same question in bacteria. Towards this end, I would like to point you towards this wiki page

It comes down to parameters such as medium concentration and hardness, parameters which we can influence. If we can extrapolate from this that cells, any cells will always follow a chemokine/nutrition gradient then we can start to understand why the system conforms to a fibonacci sequence.

Next, check out this video on a wound healing assay, what i would like to bring to your notice is that you'll find the cells replicate and move at the same rate.

If you follow a chemokine gradient, and move at approximately the same rate as your 2 billionth cousin, wouldn't you end up inevitably creating a symmetric structure?

Finally, I would, point you towards this article, where they explain a phenomenon called cell migration. This is particularly responsible for creating the symmetry which you see. And this is a similar article as the top one but it should be open access. Both support an idea which has been observed, I tried searching for the video but couldn't find it. The idea being that cells migrate collective, but there are some cells which are leaders and some followers. The leaders lead the collective migration (creating some sort of chemokine gradient) while the followers follow this gradient.

I attended a seminar last year by D Gilmour from EMBL. He showed us this particular video on collective cell migration and deposition of small clusters of cells which go on to establish the lateral line in zebrafish. You will find that a periodicity exists in this migration pattern. He relates it to a ligand activation. The related article to the video should be this one (I thought so after reading the abstract)