Still would love advice on how to program the GT to send PC messages to the ADA on scene change so i can buy a Multi box and move to using a single midi cable from the rack to the GT instead of using two cables.
Longtime MMGT and AxeFX user here. I'll offer my $0.02 on your rig - which sounds pretty awesome by the way!
MIDI Communication
For MIDI communication between MMGT and AxeFX, as you noted, you can use a pair of conventional 5 pin MIDI cables, or a get the MultiBox and use a single 7 pin MIDI cable. The MultiBox will allow you to have additional MIDI OUT ports for your MP-1 devices.
Programming and Management
I strongly prefer to use the MMGT as the single source of MIDI commands for all devices. As much as it's handy to have the MIDI block in the AxeFX, it's easier to view / manage all of the commands in a single editor, and the MMGT editor is ideal for this.
Here is how I like to set up the MMGT / AxeFX ecosystem:
(1) Remember that on preset load, you can send MIDI PC and CC commands to your devices
(2) All MMGT presets have a local page
(3) The buttons on the MMGT local page are programmed to send some combination of MIDI PC and MIDI CC commands, to control AxeFX presets / scenes, AxeFX effect block setting (channels / bypass / continuous controller setting), plus MIDI commands to control other devices. As noted above, you'll need to create your own MIDI library for the ADA devices, but again this is why I recommend doing to in the MMGT Editor. Program each MMGT local page button to send the appropriate commands to each of the devices.
In the example you outlined, I would just have the MMGT send AxeFX scene change commands, and at the same time, send a PC command to the MP-1 devices.
When I'm building a rig (for myself or a client) I start with a spreadsheet, defining the actions of the buttons on an MMGT per-song basis. Sometimes we need just an IA (effect on / off) state, and sometimes it's pretty complex where we are simultaneously changing to a different AxeFX preset / scene, changing the bypass state or channel of a block, sending a CC command as a virtual expression controller, and so on.
All of the commands originate from the MMGT and everything else in the system receives those commands via MIDI. The AxeFX sends back sysex for things like tap tempo blinking and the tuner. But otherwise I treat the MMGT as the central nervous system of the rig. It makes programming and troubleshooting much easier.