Polo bike geometry + addons

What about a classical 26" (around 40cm a-c) rigid fork, and a kind of long (15cm or more) headtube?
I really don't know if that's a good idea to base my geo around a max power fork or not (36cm).
Is the lever forces so much differents? during frontal impacts?

I just feel more confident buy basing my geo around something easy to find everywhere, but i get the frontal impact point too. Im lost.

40cm forks are easily available. My headtube is 13cm + 2cm of spacers. I kept it a little short to avoid the brake hitting the top tube. Headtube length would depend on your saddle height and how much drop you want to your bars.