I keep hearing that the legion is a "slave army" but does that mean they were slaves who formed an army... or they are an army that uses only slaves... like they take people and anex them into the army... against their wills....

I'm just confused lol

The Legion is the latter of the two examples.

The Legion conquers tribes, destroys any symbols of their individual identity, kills all those they deem unworthy, such as the old, forces the able men into the army, and keeps the women around to cook and do other things.....

Ok so they annex people and force them to be in their army, I wasn't sure because I kept hearing slave army so I thought... well then their good guys, their just slaves who escaped from slavers... but now I see thanks!

An important thing to note about the Legion is that they also have residents in their lands, not citizens, but residents.
Those are men and women who are not part of the Legion and are not slaves. Raul and Dale are two of those.
So whenever they find a town which doesn't resist and proves to be valuble and not just another tribal group they conquer it more peacefully, like Primm.
Whereas with others that are just a den of debauchery like Nipton might be erased off the map.
And finally, towns which are allied with opposing factions are conquered violently.

They also seem to go by the idea of live and let live with mutants.
They don't consider them to be "friends" and thinks ghouls look disgusting, but they won't open fire on mutants unless provoked first.

But Legion? It's all slaves, every single one of the Legion is a slave under the direct command of Caesar.
Legionnaires, breeders, priestesses, work force, all of them belong to Caesar.
But those not part of the Legion belongs to no one, but since it's still Legion's land they do hold authority and will make an example of those who step out of line, slave or not.