Justice is a fan-made global mod.

Legionaries are soldiers serving in a legion, meaning infantry, while a guardsman is an elite soldier.

I agree, and in general, why rename everyone? It's important to do it where there's an obvious mistake or where there have been radical changes to the unit itself. For example, downgrading the Paladin to tier 5 and removing his magical "lay on hands" ability, turning him into a purely physical unit – in that case, Cavalier/cuirassier/knight would be appropriate. There's room for debate about the last one.
Regarding the armored infantry – if it's elite infantry, the following terms could be used: guardsman/justiciar/crusader/zealot/swordmaster/landsknecht, but again, staying within the lore of Ashan, guardsman probably wouldn't fit (in Heroes 7, it's tier 1), although that seems a bit unappealing. Following the rule: "...where there is an obvious mistake..." (see above), a guardsman could also be used here.