You Yamaha MIDI interface will work fine with your Casio. MIDI is not brand specific ( apart from sysex ) so you should be able to get Band-In-A-Box working fine with the MIDI interface and the Casio. Just make sure you connect MIDI-Out from the PC to MIDI-In on the Casio and MIDI-Out from the Casio to MIDI-In on the PC.

Finding new tones online for your WK may not be so easy. But you can find some WK websites on http://www.synthzone.com/casio.htm to start looking for some.