For example: If an Allomancer is trying to use Allomancy in a combat but is attacked by a guard using nonmetal weapons, what can he do?
Let me see if I can make a clear description.
You have (at minimum) two participants here, though it sounds like the Allomancer is not targeting the guard, so there may be a third person (not knowing what action, it is hard to be sure).
Actions are declared in order of Wits, low to high. Since the Allomancer is having trouble with choice of action, he must have a lower Wits than the guard (or was surprised).
So Allomancer declares use of Allomancy, and receives Power Rating +/- modifiers worth of Action dice.
The guard declares a physical attack, receiving Physique +/- modifiers worth of Action dice.
The character with the largest Action dice pool acts first.
* If the guard has more Action dice: Guard chooses 2-10 dice (up to a max of all Action dice) to use for the attack. Allomancer chooses 2-10 dice (again, only up to the limit of his Action dice) to use for the defense. Both parties roll and compare results. Remainder of Guard's dice are now Defense dice. Remainder of Allomancer's dice are still Action dice and his new position in the turn order is lowered to match.
* If the Allomancer has more Action dice, he has more options:
*1* Carry through with the action: If this is targeting the guard, throwing everything in to maximize the hit may be best. If against another target, then use the minimum that you can to act and the rest will be available for defense.
*2* Skip the action and put all dice toward defense.
*3* Declare a new action: Calculate the new action's Action dice supply, but cut the number in half (round up). Allomancer is now the last person to act this round. Might be worth it if he has a strong Pewter rating and high Physique and a shield and defensive circumstances (that is to say, "new action" is an unusual choice).
- If the guard comes first, the Allomancer may change his mind and defend physically instead of using Allomancy.
If he wants, he can use all Action dice to defend and thereby forfeit his action.
- If he does not change his mind and is willing to use Allomancy, what are his options? Does he get Physique defense dices in addition to his metal,
No. The only way to add more dice is a Reaction stunt, and those don't supply from your Physique.
or he can not defend himself (like by running two steps)?
To run two steps would apply rule *3* above, so the Allomancer would end up with no Action dice and acting last in the round. This would be a very bad idea.
Or can he form defense dices from Steel/Iron
This would require either "Iron Redirection" or "Steel Deflection", two of the Reaction stunts, and already moving
metal in the area (based on the rules, pages 307 and 313). Since you said the guard has nonmetal weapons, I assume he has no metal armor, belt buckles, boot nails, or any other metal on his person.
(such as: moving metal objects to distract the soldier?)
Doing this as a distraction would again invoke rule *3*, unless the declared action was already moving an object into the guard's area, in which case I would not rule the adjusted motion a new action.