As long as your power/ground cable runs are 1/0, your big 3 is in 1/0, and your power/ground to the ZX2500 are 1/0, you should be set. Of course the 4ch doesn't need anything more than the 8 gauge.
Like others have stated, you'll want another fuse as close to the rear batt as possible. You could also utilize some sweet distro blocks as well, or instead, if you'd like to revamp your wiring, unless you haven't purchased the stuff yet?
Also, do you plan to run the ZX2500 @ 2ohms? Is your charging system consisting only of two Optima Group 34 batteries and the stock alternator? What size is the alternator?
You'll definitely want to put a 1/0 run from the front battery's negative terminal to the chassy, or just do a negative run to the rear battery. A 1/0 run from the terminal to chassy should suffice just fine for ~2.5kW though, until you plan to upgrade, then you'll want some direct 1/0 runs... //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif