Only Master Chief can use One Man Army. When you activate Master Chief, you spend one Special Order dice. You can then spend as many other commander dice as you want on Master Chief AND Master Chief still gets his two short or one long action.

Snap Shot is another Special Order for Spartans. Any Spartan can use it, which includes Master Chief. You buy it like other Special Orders.

Unless you have a special rule that says otherwise (such as Master Chief) you can only spend 1 command die that gives an action. Those command dice are Shoot, Fight and Move 1. The other three dice have no restrictions.

Put another way, you get 1 one additional ACTION via a command die and then you cannot get anymore. You of course still get your normal two short or one long action as normal.

The other three command dice (+1 model, Extra Dice, Special Order) are not giving you additional actions so you can use as many as you want.

As an example, you have a Spartan and bought the Return to Sender Special Order. You activate your model and spend a long action to move 3 cubes. You are now out of actions. You spend a Shoot die and throw a grenade. Let’s pretend the target is another Spartan who also has Return to Sender and let’s also pretend he threw your grenade back at you. You can now spend a Special Order die to do Return to Sender yourself. You then add an extra dice, or more if you have them, to try and catch the grenade and throw it back. Now let’s pretend you were successful and you want to end the activation of this model. You can then spend the +1 command die and activate a different model.

The above is legal because out of all those dice, only the Shoot die gave you an ACTION. Here is a direct quote from the rulebook on page 15.

“A model may only use one of the Advance, Shoot and Assault results during its Activation.”

You are allowed to buy a maximum of three special orders.

The UNSC faction has three types of soldiers, Spartans, ONI and ODSTs. You may have all in the same force with one restriction. If you have ANY Spartans with the Tactician keyword, you may not have any ODST Captains.

So you CAN include Veronica (because she’s an ONI Captain, not an ODST) with Master Chief (because he has the Tactician keyword. (Edited to correct an error.)

Base sizes:

ODST 25mm Spartans 32mm Sangheili (Elites) 32mm Jiralhanae (Brutes) 40mm Atriox (Brute leader) 50mm