Battle Brothers, Transports and Embarkation

So lets say that I have a unit of 5 Dire Avengers in my allied Eldar detachment and an empty Venom (using a Fast Attack Slot) in my main Dark Eldar Detachment. If I want the Dire Avengers to ride around in the Venom am I able to embark the Dire Avengers into the transport prior to deployment onto the board (and deep strike them on the Venom) or do I have to deploy the Dire Avengers and Venom onto the board and then Embark them?

All I'm seeing in the rules (under battle brothers) is the part that says: Can embark on each other’s Transport vehicles, but I see no where that states when they can embark.

Edit: Under deployment it says: "Models can be deployed ‘inside’ buildings, fortifications, or Transport vehicles in their deployment zone."

The only place I'm finding that limits units (that can embark on the transport) being deployed in a particular transport are dedicated transports that state "The only limitation of a Dedicated Transport is that when it is deployed, it can only carry the unit it was selected with (plus any Independent Characters that have joined it)."

So as long as I take up a force organization slot for the transport it sounds to me like I should be able to deploy a unit of battle brothers (given it fits the requirements to embark on it in size and rules) on a transport. Am I missing something?

If I am reading what you are saying correctly, that your plan is to use the FA slot Venom as a transport for allied Eldar then yes, that's how I read the rules as well.

Because at that point the Venom is not a 'dedicated transport' just a FA option that has room to let some people hang on the sides of it.

That is correct. I would be using a Dark Eldar Fast Attack slot for an empty Venom for the Dire Avengers, from an allied detatchment, to use as a transport and deep strike onto the board.

That's absolutely fine. Only limits on deploying inside a transport are that it must be from your own faction or a battle brothers faction and that only the 'owning' unit can begin the game in a dedicated transport. Non-dedicated transports can be used by any eligible unit.

Thank you both for the replies.