Some Fireteams can only be formed using the FTO option, as described here: http://infinitythewiki.com/en/List_of_Fireteams For example, only the Pangguling FTO option with light shotgun is a Fireteam Wildcard in Invincible Army, the other versions (EVO, Minesweeper, Totes Combi) cannot join Fireteams.
I think FTO is just the name they give to a troop or a specific loadout. It's not a rule. They just use it to differentiate a loadout that can be part of a fireteam from others than don't. But it's just a name. For example, in Varuna is a different name. The normal Clipper Dronbot can't be in a fireteam, but the Clipper Dronbot (Varuna div.) can. You just need to look at the sectorials fireteams list to know how can they link in each case.
