The "support plate" isn't needed. It does help protect the bottom of the plastic TB case against over tightened bolts, and hammered trackballs. However, if someone hammers the TB so hard the support plate "helps", it just means the rollers, bearings and bearings seats are going to take the abuse. So make sure you mount your TB tightly, but not overly tight, and don't pound the ball with your fists in frustration.

If you are mounting the trackball directly to a metal CP, no you don't need a monuting plate or kit. You'd need to drill holes in the metal though (3.25" (for the ball) and 3/8-1/4" (for the 4 bolts).
If you are mounting to a wood panel, the easiest by
far is to use either the kit or . . . the kit. Since you are getting the highball, be sure to get the highball version, and there is no highball plate, you're stuck with the highball kits (55-1103-00 or 55-1104-00).
Other things you can do:
1) Make your own plate (much like if you had a metal CP)
2) Somehow support the trackball from underneth. There are examples of people from here doing that, but it's not as easy. Hopefully they'll say their views and howtos about this.