There are many different heroes... can we assign a hero to any troops on the APC or a hero with a particular..

You can assign any of the Combat heroes to your formations (not the development ones) ad place them in any position. A few of the heroes will have a skill that benefits one type of troop so check the skill you have unlocked and if you have one of these then try to assign it to a slot with those troops. It's not essential though.