I don't see any reason you couldn't run both guitar and synth to the same looper send track. The only real advantage I can think of would be stopping/starting guitar/synth parts individually or being able to set their Pan settings apart in a stereo set up.
Unfortunately, I just got my FC-300 so I'm still in the process of learning it. If you're trying to map a pedal to turn something on/off, certain devices need the pedal set to "latch" instead of "momentary" for instance, "Device On/Off" control I had to set to "latch" on the FC-300, otherwise I'd turn on the effect on only as long as I held the pedal down (which could be cool in some situations). Hope this helps some.
In terms of playing with the band, do y'all practice to a click track/metronome? Or do you set a loop and have them play along? If you're just getting into looping, it might be best to try to just work with one loop and no overdubs while you learn with the other guys. The more time you spend with it and learn it the easier it'll be to bring the info to the bandmates.
The benefit of the send approach would be the ability to use the same looper with my guitar, the bass, and the synths. The downside of course is that the return's out is my guitar amp so those instruments would be coming out my guitar amp. If I want them sent to the PA or bass amp, I suppose I'll need to either dedicate a looper on that path, or simply assign the return's output elsewhere assuming I don't want to use the guitar with the looper at the same time.
I noticed that latch issue. It's a total bitch. I've got a memoryman delay pedal that I was planning on using the FC-200 to turn it on/off with. I'm using CC right now and they are momentary. I could use midi notes, but then I lose two buttons. I did end up buying 2 boss latch pedals that I was planning on using for this purpose. Not sure if the track enable buttons require latch, but if they do, I'd use the other switch for guitar input monitoring ON.
I hadn't thought about "scripting" the pedal pushes using a midi track in arrangement view. Unfortunately, that would force me to use midi notes instead of CC, at least I think it would.
I heard someone say Live prefers midi notes, not CC. Anyone know why someone would say this?
We don't practice with a metronome, but we also haven't been doing the live looping thing. I've sent enough click through the monitors for us to use that as a reference when live looping. The drummer's actually pretty good at playing to a click. The problem is that I just forget how to use this technical shit when it's not just me. You're right, I need to simplify if I want this to work in a group. Maybe, set session tempo by recording a 1st loop, then play everything else live except for the times I need that 1st loop. That should be pretty easy.