1) Better to make sure the S900 comes with longer legs. Do they all come with those by default now or are they special order?
When I mentioned it to my supplier back in October, they told me that this issue had been addressed a long time ago and was not a concern anymore.
2) The GH4 should only be run in manual mode (my default anyway), with ND filters (probably 3 stop) for filming (I do this on the ground too)
Yes
3) Photo mode seems to be limited to 12MP and 16x9? Unfortunate.
It is indeed
4) Some people are having problems with remote shutter in photo mode, but it should work fine if the right settings are used Is there somewhat to start stop filming (4K) remotely is do you have to start / stop on the ground?
Yes you start and stop recording from the RC
5) Some people are having trouble with gimbal balance but there was / is new gimbal firmware that fixes most issues - what firmware should I be looking for?
For what's worth my experience everyting is just fine, no jello, vibration, it's initialising perfectly and there is no problem with the balance or the horizon in flight. Don't know my firmware. Got it beginning of november. I assume it is current at this time.
6) It seems a less than perfect solution but is about the best we can expect from this level of gear at this time.
I would not say that. The GH4 is better than a 5D MIII in video, for a lot less money. The (mine at least) gimbal works great, If you do more video than stills, this setup is the one to go for. It is the choice of many...
What else have I missed?
I don't like the struggling to access the SD card and the battery when the GH4 is mounted, but apparently others are ok with it.