Welcome to the Depot MRob!
A standard size rack would be a good idea since I know the Furman and ADA's are the same depth. You always want to have room for your connections in the back without bending your connections at the jacks. I'm not sure how deep the power amp is though, so you may want to take that into consideration. Also, if the power amp is a tube amp, leaving a space between units is a good idea, or else you can use a small clip on fan in back of the rack to move air around your tube equipment. That will help prevent problems caused by heat.
When you mount your gear in the rack case, as you are looking at the rack from the back, run all your power cables on the left side, and all your audio cables on the right. Send your control cables, (Midi, foot switches) up the center. As for audio cables, try to use only the length you need for your connections. If your soldering skills are good, you can make your own cables to length, and all audio cables are shielded, speaker cables are NEVER shielded. You always want to make cables that have enough length to plug in without stressing, (no sharp bends at the jacks), and I prefer Switchcraft jacks.
I never used the ART unit, but it looks like the standard DSP processor of the day, so it will be fine in the effects loop of the MP-2, especially if you are going to use time based effects like delay and reverb. The MP-2 loops have a mix control for each loop, but I've found that if I leave the mix at 100% wet in the MP-2, and adjust the effects level on the unit to a lower, more useable mix, that the MP-2 tone is more satisfying to my ears. Of course with certain effects processors, the opposite is true. You'll have to use your ears for that.
The MP-2 is a very complicated unit in terms of programming and editing, just don't give up on it. There is plenty of help here at the Depot, and we'll get you going with it. If you are going to use the Hush unit instead of the built-in noise gate, I would recommend putting that in the signal chain between the output of the preamp and the input of the power amp instead of in the effects loop. You may want to get all your connections together and hook up everything on your desktop and audition it that way before mounting everything in the rack. That way, you'll know how everything will be connected and how it will sound when it's all in the case.
If you are going to run a stereo rig, and your 4x12 cab has the ability to run in stereo, then that would be the ideal way to audition your rack rig. Just be sure the impedance of the power amp and the cab are matched. You never want the speaker impedance to be lower than the power amp impedance or you'll cook the power amp.
That should get you started.
Harley