Watch the following Video on Youtube. You may absolutely mix calls. However, be aware that mixing driver calls will limit the ability to interchange instruments. This feature is normally possible when coding with class driver calls only. If you do not need to use class driver calls, then you may want to consider using the LabVIEW plug and play (project style) version of the driver from www.ni.com/idnet.
Up and Down Casting in LabVIEW (Mixing Specific and Class Driver Calls) http://www.youtube.com/watch?v=bPr78g7q7d4