Yes you can. As long as you can pay the Cost, you can play the Boot ability of a Z/X during your Main Phase or both Players' attack. (Well except when the opponent has Flonne up.) That said, you cannot play Boot abilities with a Sleep Cost because you would be unable to pay the Cost if your Z/X is already in Sleep.
Can you play the Boot ability of card in CHARGE position?
I assume that you meant Z/X located in Charge (Charge is a place in the field, not a position). If the ability specifically states that it can be activated in Charge (like most Start Card), then you can. If it does not, then you can't.