Like any other army the Legion attempts to reuse as many parts as possible when equipping their troops to ease manifacture. The same goes for RL armies. A pair of fatigue pants is a pair of fatigue pants no matter whether your an officer of the line or a GI Joe private.
As I see it the armors can be divided into four major groups. Light, Medium, Heavy and Templer armor sets. I have collected armors through-out the game and this is a bit from my showcase.
All in all the armor reflect what I see on members of the Legion in game, and I presume this is the general outfits for legionaires throughout Tamriel.
Studded Leather
Newtscale Leather
Newtscale/Dragonscale Hybrid
These armorsets are very infrequent in Morrowind, presumably because it seems to be standard issue in the Navy alone.
The guard on the slaveship is wearing such an outfit for instance. Another plausible explanation is the use of genuine leather which cannot be produced in Morrowind, and thus the use of the native Netch leather is preferred to the imported leather. The armors seems to reuse Imperial Chain for helmet, greaves and pauldrons. I have not seen it, but Newtscale could be worn with Dragonscale instead of Chain for helmet and possibly shield. There's no standard issue light or medium shields, and these troop does not seem to wear shields at all. Naval troops must have both hands free as it seems.
Chain
Dragonscale
Chain is a standard issue armor for light shock troopers in border areas (such as Fort Darius) and for other light troops such as archers. This armor reuse standard issue Steel for shield, gauntlets and boots.
The Dragonscale Armor replaces helmet, cuirass and shield with armor of said making. It seems very infrequent which leads me to the conclusion that it is predominantly used by special honour troops, possibly Templars, but this cannot really be confirmed.
Steel
Silver
Duke's Guard
Imperial Steel is the only truely complete set of Imperial Armor and must thus be assumed to be the standard issue armor.
This armor is used by guards and troops in major forts and cities.
Certain honour guards (e.g. in Ebonheart) replaces the helmet with Silver Helmet and either Silver Cuirass or Duke's Guard Silver Cuirass
I fear that Sellus Gravius is going to die soon :-)
Patience is a virtue to some