It might be worth building the background on to a sheet of plastic or something, that way you can remove it and reuse it if you change your mind, something happens to the tank, etc.
If it's not actually glued to the back of the tank, I'd coat the stryofoam with something, even just one thin layer. eventually, it will break down and you'll end up with little beads or bits of plastic fluff in your tank.
As to the filter siphons- are you using a HOB powerfilter? easiest would probably be to just hang one on the tank and measure how much room it needs. If you are feeling ambitious, you could even work it into the background design, and have the uptake tube concealed, so that the filter uptake drops into a larger tube embedded in the background, and that tube connects to a mesh/sponge covered uptake hidden in a pocket of the background.