Walkthrough tips, Discussion of Campaigns [Heroes 5]

People, HELP!!!!!! I've almost completed the "Ship in Inferno" mission in Heroes 5, but I still don't understand if it's possible to split an army between heroes (i.e., divide 100 Imps into 99 and 1)???? and how is it done????
Because I always either give the entire unit slot to the main hero or nothing at all!!!!!!!!!!!!!!!!!

I REALLY NEED HELP.