Important employment considerations

You are an HR manager, what if your organization has been successful on a major project in which not only full-time employees but temporary workers from a temporary agency have contributed, and Henry Simms, the Plant Manager wants to reward everyone who worked on the project alike? The project consisted of several strong contributors that did the majority of the work. As the HR Manager, are there important employment considerations you need to discuss with him? Is this a wise decision? How would you recommend this be handled? Would this establish a precedence?