Thanks Chris!
That works perfectly for the dissolve_field. When I try to set the statistics_fields like this
DissolveTool.statistics_fields = "instln_id,{FIRST}"
I get this error:
Failed to execute. Parameters are not valid.
ERROR 000315: Statistic field instln_id must NOT be of type TEXT unless methods are FIRST or LAST
Failed to execute (Dissolve).
John