I'd suggest having two ranks for each faction: Minor/Major along with Technological Achievement Tiers.
By technological achievement, I propose the following which are inspired by Halo Lore, from lowest to highest:
Tier 7: Stone Age.
Tier 6: Industrial Age (Factories and such).
Tier 5: Atomic Age (The Splitting of the Atom, WMDs, etc).
Tier 4: Space Age (e.g. the Moon Landing).
Tier 3: Space-Faring (Semi-Sentient A.I, discovery of FTL, colonisation, etc).
Tier 2: Interstellar (Pinpoint or "super accurate" FTL navigation, near-instantaneous interstellar communication, application of energy manipulation (such as plasma weaponry))
Tier 1: World Builders (Ability to manipulate gravitational forces, create A.I with full sentience, creation of super-dense materials, create life and create worlds.
So the idea is that a faction may be minor or major depending on the amount of writers behind them or their role in the plot, but even minor factions would be able to excel major factions in terms of tech.
For example, right now, the UNSC as a supposedly Major Faction would stand at the latest stages of Tier 3 or the earliest stages of Tier 2. However the Forerunner Remnant, supposedly a Minor Faction, would stand at the highest stages of Tier 1.
I propose that the "Tiers" should be designated to factions depending on the description of their technologies or capabilities.