The answer to your problem is really very simple. The MIDI-specs define this thing called a Program Change. (Often called Patches, at least when dealing with GM or soundfonts). Most midi-keyboards already have buttons to select various the various programs (instruments) available, and FL responds to that out of the box, no extra automation or mapping is needed. If your keyboard is programmable, you will probably be able to make the buttons send program changes to predefined programs as well, so that you won't have to go through several programs to find the one you want.
wow, I hadn't realised before, but what you're saying is true. It works a bit awkwardly ( I have 10 selection buttons and they work as 4 only), but it works.
thx!