OK - gotta get this bit outta the way. Search, noob!
Right, now that's done, lets move on...
1. I'm not sure.
2. Twin tubes offer better mechanical protection to the working tube, but have poorer heat dissipation. Fade is generally caused by overheating, so you could argue twin tubes are more likely to fade than monotube - all else being equal.
Hard/soft, short/long - it all comes down to the individual shock.
3. Larger bores size means more volume of oil for a given stroke and larger surface area for heat dissipation, so probably less likely to fade. I presume larger piston area means they can do fancier tricks with valving too, but I don't know if that happens. The largest bore you can fit may be limited by your suspension design - you may not have room for larger.
4. Many people have had progressive springs sag quite early, so prefer linear springs. A progressive spring will supply more droop travel than a linear spring, which helps with articulation - if nothing else limits your articulation first. The most obvious benefit of progressive springs is the ability to have a softer spring when unladen, and a stiffer spring when loaded for trips. Because the progressive is softer when unloaded it will compress more than a linear spring would when loaded - so the difference in ride height between loaded and unloaded will be greater. If you will ONLY ever drive your car loaded, linears are probably a better option. If it's a "one car does all" situation, I'd go progressives every time. (I did, and they're still fine...)
5. The spring rate you require depends on the weight they need to carry, and what you want to do with it. Softer will normally give better articulation, along with more body roll. Harder carries weight better but will lift the body sooner when crawling over big rocks/wombat holes. Essentially, it's horses for courses, so a suspension design will always be a compromise. Decide what's important to you, and tailor your suspension accordingly.
6. Shock absorbers need to be selected to suit the springs and suspension travel. Adjustable shocks allow you to fine-tune the damping on the car, and even adjust it for different road conditions - soft for slow off-road work, stiffer for highway driving.
Let the arguments begin!
Scott
