With that particular message, I would double-check the columns going into each side of the UNION. If they match up correctly and it is truly the case that the same field is a smallint on one side, and a character-type field on the other side, then prior to the UNION, consider replacing the smallint field type with a character-type (e.g. use the In-DB Formula tool to create a new character field that does the simple cast to a character; then a In-DB Select tool to deselect that smallint field and replace it's position in the data with the recasted character-type field. Hope that makes sense: I can't create a workflow sample workflow for In-DB.
