Well, the first thing I'd say is that you can build your rig to work with either the SLO or either of the Mesas. IIRC, they all have line/slave outs.
To me, it seems like the choice of running a single
Marshall cab for the dry in the middle and using 1 Mesa cab on either side, making a W/D/W rig is going to sound the best. In a W/D rig, reverbs are going to sound weird coming from only one side, and you'd be missing half the imaging from panning or ping-pong delays. If you're going to gig with a W/D rig, you'll either have to sum to mono FOH or half the audience isn't going to hear your effects. Those are the reasons I'd recommend a full W/D/W rig.
Since your power amp is stereo, and you've got three cabs, so sholuldn't need anything else along those lines.
So, with your GCX, you could use loops 1-4 for your OD, flanger, volume, and wah. After that, you'd go back into the amp with a dry cab connected and the line out running back to GCX loop 5.
One thing... I'm not sure if the GCX always sends the input to the output of each loop except when there is something plugged into the return. I think it does, but I'd have to double check the manual. If it doesn't you could use a buffer or splitter with 1-in/4-out, or 3 1-in/2-out buffers. I know JHS makes a good 1-in/2-out buffer and I'm 99% certain that Analog Man would add extra outputs to one of their buffers. Of course, you could use your Rane for this, but I think there's a better use for it later.
OK, the send of loops 5-8 would be sent to 4 different processors. Any 4 will work, so it's entirely your decision. However, based on what you have, here's what I'd probably pick. And of course, you could change your mind at any time, since all you'd have to do is pull one out of the rack and put the new one in it's place. Anyway...
5 -
Ibanez AD202
6 - Rocktron Prorax Chorus
7 - TC Electronic G-Major
8 - Lexicon MPX-1
You can turn each one on by enabling the corresponding loop and you'll get tails from any time-based effects (delay, reverb, etc.).
From there, you'd run the output of each processor in stereo to the Rane SM26S. Using the "Main Input" gives you 8 channels. While that channel is already stereo, don't forget to set the other stereo pairs to pan hard right and hard left as appropriate.
Run the main output from the Rane SM26S to your Velocity 300 power amp (again, set to stereo) and connect each side to one of the Mesa cabs. You can still get a switchable analog dry path through to the wet amps simply by bypassing the Ibanez AD202 (assuming that it's all analog, which I believe it it). Another option would be the Rocktron Prorax Chorus, but only if it is all analog.
What this doesn't give you is midi control of your amp's foot-switching functions or a MIDI pedal to control the rig. For MIDI control, I'd recommend either a RJM Mini Amp Gizmo or a Voodoo Lab Control Switcher. For a MIDI pedal, pretty much anything will do, but I'd look for one that can send Continuous Controller (CC) messages as well as program change messages. It'd be even better if it had either one or more built in for jacks for external foot controllers (EB VPJr would be my recommendation, but there are lots of options), or built in foot controllers. Both the TC and especially the Lexicon can greatly be enhanced with continuous controllers.
Another alternative for channel switching is to not use one of the loops 5-8 for a processor but for a switch. I know you'd mentioned that, and you're right, it would work just fine. You could have any combination of processors and switches that best suited your needs. Just keep in mind that, if you want dry signal in the wet cabs, you'll need one loop for that if you might ever want to switch it off and another loop to use as a bypass switch if you don't want to leave it on all the time. Of course, if you don't care if you can turn off the dry in the wet cabs, you can just add it to your Rane SM26S and use the loop for switching.
If you wanted to change this to a W/W rig, I would either use one of the Boogies with the power amp turned off, or the Boogies or SLO running into a suitable load box. The Suhr reactive load gets a lot of love for that, as does most all of the 2 Notes products. There are other options here, too.
Anyway, I've probably rambled enough tonight. I still owe you the pics and diagrams... hopefully, I'll get to them tomorrow.