The pairing you cannot take. Throne-bonded Outriders (1 DP) plus Spearhead-at-Arms (2 DP) is exactly 3 Detachment Points and is illegal, because both carry ARMIGERS and this detachment's own printed rule forbids it.
We can state that flatly because both halves of the chain are first-party: the restriction is printed in the pack, and Spearhead-at-Arms' ARMIGERS tag is printed in the Munitorum Field Manual v1.3. The two official documents do not disagree; the pack simply does not mention Spearhead-at-Arms at all. One piece of contrary evidence, disclosed rather than buried: the community mirror's Spearhead-at-Arms entry prints no tag sentence. That mirror is one version behind the Munitorum Field Manual for this faction, so it is a lagging document rather than a second opinion. Confirm in the Warhammer 40,000 app before a tournament.
The pairing people wrongly avoid. Dominus Foebreakers plus Questor Forgepact plus Throne-bonded Outriders is 1 plus 1 plus 1 equals 3 DP and is legal, even though one of the three carries a tag. Carrying a tag is not the same as sharing one. The restriction only bites when two detachments hold the same tag.
Recommended pairings, both unambiguously legal:
Plus [Questor Forgepact](/40k/armies/imperial-knights/detachments/questor-forgepact) (1 DP, total 2). Armigers hunt, 500 points of Skitarii hold.
Plus [Gate Warden Lance](/40k/armies/imperial-knights/detachments/gate-warden-lance) (2 DP, total 3). The big Knights anchor a defensive line; the Armigers roam forward under a Bondsman ability. Neither rule competes for the same models.