To have multiple synths, you need the 2012 version of Band-in-a-Box, otherwise use RealBand.

Have one syth setup as your default. Then, for other sythns, open the Mixer window, and then click the Plug-in button. You can choose a synth for each track from that window. (The mixer button is next to the Play button.)

I can't answer about the potential latency problem.

Jim