Heres my take on the different ranks for the prometheans. The crawlers have the normal ones and the spiky tougher ones. The knights have two variants, normal ones who usually have scattershots or incineration cannons and black ones which have binary rifles or incineration cannons. In general the knights are a lot tougher and act more as high ranking enemies for the prometheans, equivalent to the white and gold elites. The soldiers have 3 variants, the normal ones which pretty much only carry suppressors, black ones which carry light rifes, and big spiky ones which carry splinter turrets or scattershots. These 3 ranks seem equivalent to the elite minors, rangers, and majors respectively.

For the covenant, well I've given up on ranking those so I end up prioritizing them based on weaponry. Also, it seems that the different ranks have less impact on their deadliness, with a gold elite being just as powerful as a major or minor.

In conclusion, I agree that they need to vary covenant armor more and maybe even make the different ranks have a noticable difference on the battlefield. Since they added the prometheans and especially in Halo 5 the covenant just doesn't feel as fleshed out and interesting to fight.