Is this gamey?

[FLG]

I would say probably not. It would depend on the reason the unit is fixed. If they are awaiting historical orders to enter a battle and the enemy moves up to them, then it is reasonable to assume they can act to defend themselves.

If the designer really doesn't want to the troops active until a given time he can arrange for them to arrive as reinforcements. This is done with the artillery units of the Stalingrad Front in the main Stalingrad campaign. The Stalingrad front didn't enter the battle until a day after the Don Front. The designer has positioned all the Front units except the artillery pieces which arrive as reinforcements a day later.