Im pretty new to Conquest, and had a few things my buddy and I could not find out in our read through of the rules. I checked the boards, hopefully these aren't reposts. Most of them are based on Actions and their timing.

Can you take a Combat Action at a planet where you have no units?

How many actions can each player play during an Action window?

Do they play them in initiative order?

If I pass my first chance for an action in a window, and then my opponent plays can action can I then play one in response to that?

Can I play an action in the combat phase action window to play a new Unit such as with Brother Maxos and then immediately attack with that new unit?

When in the Combat Phase can you use an ability like Ambush. Does it take the place of your attack for that round or would it occur in the action window?

Can Tzeentch's Firestorm target a unit anywhere on the board? And if an opponent plays a shield card can the Chaos player pay more credits to boost the damage?

Yes, you can take a combat action at a planet where you have no units!

There is no limit to the number of actions you can take during an action window.

You take turns taking actions during an action window, the person with initiative gets to go first.

Actions aren't taken "in response", but yes, if you pass, and then your opponent plays an action, you can take one after his resolves.

Yes, if you use Brother Maxos's ability during an action window to play a Space Marine and then the action window closes and it is your turn to attack, you can attack with the unit you just put into play.

Ambush is an action, so it takes place during an action window, it does not take place of your attack. Action windows happen between attacks.

Tzeentch's fire storm can target any legal unit on the board. And no, you can't pay more after a shield is played to increase the damage.

Im pretty new to Conquest, and had a few things my buddy and I could not find out in our read through of the rules. I checked the boards, hopefully these aren't reposts. Most of them are based on Actions and their timing.

Can you take a Combat Action at a planet where you have no units?

Yes, as long as the specific action in question doesn't require you to have a unit there.

How many actions can each player play during an Action window?

As many as you can manage; but you and your opponent alternate having the option to use an action until you both pass consecutively.

Do they play them in initiative order?

The player with Initiative for that combat has the first opportunity to act, then alternate (see above).

If I pass my first chance for an action in a window, and then my opponent plays can action can I then play one in response to that?

Yes.

Can I play an action in the combat phase action window to play a new Unit such as with Brother Maxos and then immediately attack with that new unit?

As long as it is your turn to take a Combat Turn next, yes.

When in the Combat Phase can you use an ability like Ambush. Does it take the place of your attack for that round or would it occur in the action window?

The action window. Actions are not the same as Combat Turns.

Can Tzeentch's Firestorm target a unit anywhere on the board? And if an opponent plays a shield card can the Chaos player pay more credits to boost the damage?

Without checking the card I can't remember if it has a targeting restriction. If it doesn't specify otherwise, then yes. As for shielding; no, you're now past that point in the resolution, as x was determined when you paid the cost.

Thank you guys for the responses. Just one follow up

Ambush isn't listed as an action on the Eager Recruit , but it is still an action?

From the RR entry for Ambush:

A player may, as an action during the combat phase,

deploy a card with the Ambush keyword from his hand.

emphasis mine.

Thanks again!

@CommissarFeesh: the only restriction on Tzeentch's Firestorm is "non-Warlord unit". No location or timing restriction.