Reading the annex it appears as if fire team duo is allowed within vanilla. "► Both members of the Fireteam: Duo must belong to the same unit, or those combinations of units indicated on their Sectorial Army List." Compared to "Core: ► Only those units specified on their Sectorial Army List can compose a Fireteam: Core. Haris: ► Only those units specified on their Sectorial Army List can compose a Fireteam: Haris."
Don't know what you're on about: REQUIREMENTS ► The Trooper possessing this Special Skill must belong to a Sectorial Army List, or have a Special Skill, piece of Equipment or Hacking Program that allows its use It's pretty clear if you're not playing with a sectorial, you can't form Duos.
What Icchan said. TeamPro fulfilled ‘Hacking Program that allows its use’, but TeamPro no longer exists to be able to do that.
Out of curiosity I did a scroll through the vanilla armies looking to see if there was anything that might be broken. There's a couple heavy TAG duos that are probably pretty sweet but massive investment in things like double Guija or double Squalos, and Ariadna and O-12 would probably both elect to duo their varangians just to be able to take cover until they wanted an impetuous order, but... that's it? Like, duo mukhtar in Haqqislam would be pretty sweet, and gecko duos in nomads would be cool, but there doesn't seem to be a balance issue. Ahhhh well.
They are still working on the Fireteam-Rules. Dunno why they were not able to finish them before releasing N4, but I suppose once they release an update of the Fireteam-Rules this "hole" in the rules will be closed as well. Either that it says it's not possible to form Duos in Vanilla because the part with "or have a Special Skill, piece of Equipment or Hacking Program that allows its use" will be deleted or that they insert a possibility or rule to form Duos in Vanilla.
Now that they removed Tinbot from the Yan Huo I don't think there's even any interesting duos, let alone powerful ones, except Adil who is listed as Wildcard in vanilla Yu Jing, of course (similar to Krit + Guijia special Duo being in N3 vanilla Yu Jing)
Given the N4 default format of 15 troopers, giving Vanilla lists the option to form 1 Duo doesn’t seem too terribly broken, and provides a bit of order efficiency that’s otherwise lacking compared to Sectorials, no?