Starteur,
You can use either the o rings, 4 or 5 in each shock or 5mm of fuel line. As Tony stated you need to ensure that you put grease on the inside of the black rubber tube otherwise it does not slide properly. The tube can be cut shorter as stated in a previous post. 8mm front and rear. The o rings do not have to be black, they can be orange etc, just as long as they are the same thickness. You can still use the standard spring that came with the kit.
The other springs you have are the short TRF springs. Tamiya part number 53333 spring set will still work with your modified shocks.
The standard springs will work, however the ride height is still a tad high. All of those that I have performed this modification for have been pleased with the results. The main thing is to use plenty of grease. The same grease you get in the kit.
These shocks will not be as good as oil shocks, but can be made to work well. If you really want them to hook up, then you need to measure the springs so you can match them. Now we are into another dimension.
Far easier to get the super mini cva shocks.
Regards,
Calvin.