I don't know if the Roland synth you use is GM compatible, but you probably should always start off using the default instruments found in the style. I very rarely change the patches for job they did with the instrumentation in the BIAB styles are very good. Occasionally, I'll substitute Acoustic Bass for Electric Bass but most of the time, I leave the patches alone. Changing a patch can lead to some 'interesting' sounds like substituting piano for jazz organ, etc.

To answer your multi-style question, the answer is yes. You can mix and match parts from styles using the Hybrid style maker. It's pretty easy. Just make sure your feel and tempo of the styles you are merging match.

Good luck,
Rachael