Let me say that I don't use an outboard mixer with my setup, I use an aardvark Q-10 soundcard that has a built in mixer with the driver/aardvark programming.
That being said, I have seen other peoples setups and you can use your 16 channel mixer, but it doesn't have to do so much with SX as it does your soundcard. If your soundcard only has one input then your not going to be able to fly in 16 different channels at one time just because you have 16 on your mixer. Think of your mixer as a way to control the sounds that go into your soundcard, then sent from your soundcard to SX to be tracked, then SX's mixer is what you use to automate and manipulate the tracks.
SX has a mixer built into the program. Fully automatable with automatable effects, automatable well...just about everything.
There are consoles/controllers that let you automate from them, and there might be a way to hook yours up, I'm not sure. Blue Bear is very knowledgable about systems and he might be able to better answer your question.
Where does this hook in with VST?