Civilization – What are the XP levels for units in Civ 5

Unit stacking is allowed in Civilization, its just restricted.

"in Civilization 5 there is no stacking of units" is an oversimplification. From the Manual:

Units are subject to “Stacking” limitations – two military units may not end their turn in the same hex, nor can two non-military units, but one military and one non-military unit may end their turn stacked in the same hex.

Because of this, there are two cases in which units can become stacked:

1. One combat unit can be stacked with one non-combat (civilian) unit by normal movement or other means like below. (And they may end the turn this way.) This means your worker is allowed in the same square as your warrior, which is important for defending your vulnerable non-combat units.
2. When you build (or buy) a new unit in a city, it is initially stacked with any units that were already in the city. I'd guess, but have not yet seen, that this can also occur due to receiving a new unit through other means, such as ruins or a city-state gifting a unit to you.

When a unit in the first situation need orders, the game uses the usual message "Unit needs orders" because nothing is wrong. When a unit in this situation needs orders, the game uses the special message "Stacked unit needs orders" to let you know you MUST move a unit.

The supply limit based on your empire size is not related to the upkeep cost. Instead, if you exceed the supply limit, all your cities take a 10% production penalty. In most games, you will run out of money before you get to that point.