My problem with the hold fire command... so they will only return fire? What if i want them to hold fire till the enemy is danger close, ambush them in surprise... then should I be leaving fire mode on or hold?

If you have them in hold fire mode they will ignore the enemy until you take them off hold fire mode, or until the enemy is in the square next to them where they will ambush them, or until the enemy fires on them at which point they will fire back.

I've found hold fire mode very useful to either ambush or let enemy pass by so you can get a shot at the weaker side/back armor (or for suppression recovery) very much needed especially when trying to handle a superior force while you move other units in for support.