Can only guess at the number of O rings to use under the piston, since I don't use them. If I remember correctly, they are 2mm in thickness. Can tell you this though, use only the black o-rings. The others are too soft. I'd start with 2 and measure, Usually, the easiest measurement is to measure from the start of the threads to the bottom of the shock end cap. That should be 6 to 8 mm. That will give you a shock length of 56 to 58 mm, usually.

The aluminum spacers will give you the ability to adjust droop, which is done with shock lengths, in 0.5mm increments. Also, pack the springs that came with the shocks away somewhere that you won't be tempted to use them. They are much too stiff. The #53333 spring set is the one to go with. Put the blue ones on the rear, the reds or yellows on the front.

Use the 3 hole pistons and 40 wt Losi oil all around. If you're using Associated oil, the equivalent is 30 or 35. The Tamiya yellow oil is about the same as the Losi 40. A lot of what you do with the shocks and springs depends on which motor you're using. I struggled for a long time trying to get the 21.5 to work with a car set up for the Sivercan. the shocks were totally different.

Please remember this info is just a starting point. Incidentally, rear roll bars are beneficial 90% of the time. Front roll bars are for the advanced Mini drivers only. Another point to remember is that SWB cars set up differently from a MWB or LWB car. My suggestions are only for the 2 longer WB cars.