1. I need to extend my fuel pipe from the tank (never been any fuel in it) it is 10mm dia. i want to reduce it to 8mm, so i was thinking of using solder ring fittings. Will solder perish in the fuel line.

It will be fine - my weber carb drillings are plugged with solder and they have lasted 27 years!

Fuel lines on my car have soldered on olives, as hose clip securing lips, been on for two years now, no problem. Ray