The springs are just to dampen the transition of power through the disc. In the end 4 or 6 should have the same strength overall.
As for the diameter, the disk has to be compatible with the FW and the PP, specifically it has to fit inside the PP housing when mounted. The disc and PP usually come together. A 1/16th of an inch won't matter.
Here's a Bully Stage 1 picture.
As for tools...join a local car club as many have community tools that you can borrow and possibly a few guys could help you out.