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I'm about 95% that I'm doing this right but I'd still like to get some confirmation.

I'm interested in the "Endless Swarm" Formation out of Rising Leviathan. It very much suits my gaming style, with tons of creatures and scaled-down monstrous creatures (i.e. no wings, stock weapons, no double devour on everything etc.) to afford them.

The way I understand it: to remain within the boundaries of the FoC I'd need to select a primary detachment, which in turn needs 2 troop choices. I can then select the formation as a separate detachment on top of that.

While I feel like this (most likely) how it plays out I still have my doubts. It leads to a game situation that is a) incredibly unfluffly and b) bookkeeping intensive, as I have to keep track of two entirely different set of troops - one has the Endless Swarm special rule, the other one Objective Secured. I pretty much have to take Gaunts for the mandatory troops in the primary detachment (it's not like I have much of a choice) and since the models look exactly the same, confusion is bound to occur.

I feel like this formation is probably better fielded in an unbound army, so that I don't have to take 2 extra mandatory troops choices on top of the formation unit choices. What do you guys think about that?

In 7th, you are not required to take what in 6th was known as a "primary detachment" - The old FOC is now a "Combined Arms Detachment", and is not a requirement for a legal army. Your Primary Detachment is simply whichever detachment has your Warlord in it.

A legal Battleforged army in 7th is made of any number of Detachments in any combination. A Formation is simply a specific type of detachment, so you can have an army of nothing but formations and it will still be legal.

As above.

You can have a legal Battleforged army that is just a single Endless Swarm Formation (or more than one), as a Formation is a type of Detachment.

The "boundaries of the FOC" as you describe no longer exist. The FOC is now just part of a detachment, each detachment has their own unique FOC (Formations none at all).

So yes, you can take Endless Swarm and if you have enough points left you can take a second one.