To answer your questions Vredesbyrd, I do not believe it would be appropriate for Garro to wear an older mark of armour with the ornate cuirass and head eagle. From my understanding, this honour was reserved solely for the honour-rank of Battle-Captain of the 7th Company. This ceremonial piece was only worn by the Battle-Captain in combat or upon formal occasions.
According to the novel ''Flight of the Eisenstein'' the ancient Power Sword ''Libertas
'' had no visible imperfections visible in the crystalline matrix of its monosteel blade. It was said that this ancient sword pre-dated even its bearer. Some elements of the weapon had been fabricated on Old Earth before the Age of Strife. Since it sounds like this was a master-crafted weapon, I highly doubt Garro would have possessed either such a fine weapon or the ceremonial cuirass as a line Astartes. He more than likely acquired these items from the former Battle-Captain of the 7th Company once he was raised up to take command as its new commander. This was common practice to pass items on from one commander to another.
In regards to the honourific "Battle-Captain," while many Legions had the tradition of giving the honorific of "First-Captain" to the commanding officer of the elite 1st Great Company, the Death Guard also held two more privileged titles, to be bestowed upon the leaders of the 2nd and 7th Great Companies, respectively. Thus, although they held no actual seniority over one another, Captain Ignatius Grulgor of the 2nd Great Company could carry the rank of "Commander" if he so wished, just as Nathaniel Garro, as Captain of the 7th Great Company, was known as "Battle-Captain."
Here's a pic of a Dusk Raider Legionary in their pre-Death Guard colours:
Dusk Raider Legionary