Sounds like fun, but that's gonna be a helluva lot of RCAs to run from the front to all of the amplifiers in the rear, and a lot of speaker wiring as well.Thanks. I actually do think they'd look cool if they were mounted side by side in the back. My only issue is where to put the DSP, but I think I can fit it up front under the steering column or behind the glove box.
You might want to replace just one of those amplifiers with a good combination DSP/Amplifier unit to keep at least 90% of the RCA signal wiring short and confined to the rear with the amps. It might also be nice to have that "spare" amp set aside "just in case".
EDIT: Sorry I missed that you already have a miniDSP 8x12. Great choice, but I would still try to mount it with the amplifiers if possible.
I would use a relay if you are going to power multiple cooling fans, and keep them on a separate circuit from the amp remote turn-ons. HOWEVER, barevids on YouTube (who is an amplifier repair tech) posted a video showing that the remote turn-on circuits within 95% of the amplifiers that are made need WAY LESS power (current) than a standard Bosch automotive relay that is typically used for this purpose, even when you need to turn on 8 or more amplifiers!
But if you do use a relay, the "Quenching" Diode is definitely a good idea.