Quick answer for your question - you can use Drools Guvnor for both business rule and process authoring.To evaluate one ore more business rules during your process execution you set the Ruleflow Group attribute of your Business Rule Task, for example:
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-756874-19326/450-373/1.png
and in your rule(s) set the same value for its ruleflow-group attribute:
https://community.jboss.org/servlet/JiveServlet/downloadImage/2-756874-19327/450-127/2.png
As far as using drools DRL syntax in sequence flow condition expression, you can find that in the jBPM documentation.
Also read more about how processes and rules interact in Mauricios great blogs:
http://blog.athico.com/2012/07/processes-rules-or-rules-processes-1x.html
http://blog.athico.com/2012/07/processes-rules-or-rules-processes-2x.html
http://blog.athico.com/2012/07/processes-rules-or-rules-processes-3x.html
HTH
