Hi Carl,
using nine different Synths does indeed sound unusual. But I guess if you are using completely different setups for the synths (like different reverb and chorus settings, different Soundfonts etc) it might make switching in a live situation easier / faster.
As Tom has already said, channel volume can be controlled via MIDI CC 7. And if you are looking for a nice user interface to implement sliders and other MIDI controls, TouchOSC might be a good candidate:
Then you could use touchosc2midi to pass control messages from your Android tablet to fluidsynth:
It's a bit fiddly to get setup just right, especially as MIDI is a second-class citizen in TouchOSC (which mainly focuses on OSC, as the name implies). But especially the nice user interface to create control surfaces really makes it worthwhile, in my opinion.
If I can remember correctly, touchosc2midi can connect to multiple midi ports (i.e. each of your nine synths) and in TouchOSC you can target those ports individually.
Also, note that there is the option to enable a Telnet port for Fluidsynth. That way you can send configuration commands (like selecting a sound from a specific Soundfont) via a socket. That might remove your need to start multiple synths in the first place.
And last but not least: assuming your keyboard sends on a single channel, you could also use a MIDI router to distribute those messages to all MIDI channels. Either use Fluidsynths built-in router (which works great but isn't very well documented), or use an external router, like mididings
http://das.nasophon.de/mididings/
Then you could use a single Fluidsynth instance and set up your sounds on individual channels. Your keyboard will play all channels simultaneously and you can use channel volumes to mix (or mute) them to taste.
Cheers,
Marcus