I havent run mine FS200 yet but running other OS engines can answer your questions.

Mounting in any position is fine as long as the tank is centered with the throttle drum.
It does not need a pump unless you place the fuel tank fare back from the engine and on the CG.
I think 18x6, 18x8 or 19x6 are very good for 3D.
Dampers are not needed, usually weight too much and can be the cause for other problems.
All OS engines run just fine on 5-10% nitro but 10% or more is better as usuall.