Origin: The Alliance were conducting experiments to make the ultimate super soldiers, their experiments went awry when the Genetics went unstable, and the insane Juggernauts broke loose and now roam free throughout the Galaxy. They've come to call themselves the Melchizadeck and consider themselves guardian angels, some cultists even believe them to be angels in the form of giant cyborgs. They build their own ships and their own weapons. They are as much a race as they are an organization.
Ranks/Lifecycle: Melchizadeck go through several rather unorganized ranks in their military service, ofcourse they don't consider it the military they consider it the path to enlightenment.

1. Carriers (Carriers carry the unborn Melchizadeck in pods)
2. Pods (The pods will fly off of the massive Carriers and attach themselves to birth towers, where they will protect the Nymphs until they are born.)
3. Nymphs(Nymphs have what seems to be a baby's head but they have massive arms, and due to their genetics they have advanced evolution, and have evolved in the past decade to fly because of thier underdeveloped limbs. When they're mature enough theyll fly away.
4. Adult/Drone(The Nymph will lose its wings once it is able to walk, and will shed as it grows. Once it is mature enough it will be given its armor, and the equipment it needs to build its weaponry, surprisingly the knowledge of combat and bulding equipment comes naturally to this war-bred race.)
5. Plague Walker Plague Walkers have the ability to breath an airborn virus that will mutate its enemies. Drones are given this ability when they're old enough by the Brood Queen.
6. Boulderdash Older Plaguewalkers or Boulderdashes have the ability to roll up into a ball and pummel through their enemies.
7. Juggernauts Juggernauts are larger and more mature versions of the Boulderdashes, they have lost their ability to roll up into a ball but have extremely hard suits that resemble diving suits, but are developed for deep space travel.
8. Titans Titans are massive, standing over 16 feet and weighing in at over 5 tons, Titans are extremely unstable behemoths mutated by the Brood Queen and Primadonas to defend the Brood Queen.
9. Primadona Female Melchizadeck, as bueatiful as they are, are mutants that are born pregnant, as they grow older they become the much more grotesque Brood Queen.
10. Brood Queen The Brood Queen is a monstrocity of flesh, and eggs. She is attached to the floor of her nest and her only purpose in life is to command the armies of the Melchizadeck and reproduce. Carriers will then collect the eggs and place them in pods, partly living capsules that protect the eggs in shells harder than diomond. There is only one known Queen in existance, Christian Priests believe her to be a manifestation of the virgin mary, and therefore prefer to call her 'Mary'.

Weapons:
Even though each unit of the Melchizadeck army had a different weapon, a few specimens have been captured.

Water Vapor Gun.
The water vapor gun collects water vapor and freezes it, this will instantly freeze the enemy, also called, the Freezethrower.

Giant Drill.
A simple drill-like weapon, this mangles the enemy brutally.

Anti-matter Gun.
This gun focuses a painting laser on an enemy and an Anti-Matter cannon that the Melchizadeck have placed around many planets, will eliminate the painted target from existance.

CP Rifle
This Assault Rifle fires chrystalline projectiles that are harder than diomond. Most Plague Walkers use this.

CP Pistol
This is a standard sidearm that most Drones use.

Theron Staff
A staff made of flourescent foriegn material that when hit by it the target's wound will slowly begin to grow and desintigrate the body, it is still unknown what this is made out of. This is seen mostly weilded by Titans.

Comments are welcome.

How do they feel about the citadel and the alliance?

They don't really care about them as long as they do not get in their way. They're not allies nor are they enemies, but they can be enemies, most of the time in battle against the Melchizadeck, The Alliance will suffer from too many losses and have to either pull out or move to air combat. When in battle against the Citadel forces they are mostly evenly matched, but their troops are very powerful individually so even a Melchizadeck scouting party can prove troublesome to defeat.

Thanatos, in order for this to work you'll have to create some sort of political controversy. Make it that there's an ethics debate about Melchizadeck amongst humans. With the political pressure, it will prevent the Systems Alliance from simply nuking them from orbit.

Their powerbase will have to be small and they'll most likely have to rely on asymmetrical warfare to survive. Without infrastructure, any other species can win by simply outproducing weapons and ships, regardless of physical ability.

In that case, commerce raiding will be key. Picking off unprotected traders for cargo. Otherwise, have them hired out as manual labour or bodyguards as a legitimate enterprise.

I should just make it to where the mount of space they control is so vast that all attempts to attack with even the largest fleets hve proved useless. They also have some sort of as of this moment, "unknown" technology that makes their cities 'immune' to nukes. They are not very political, in other words, they are a very outright simpleminded race of beings.

That still doesn't work. More space means that forces are spread thin and a task force can divide and conquer, attacking systems piecemeal.

As for the anti-nuke thing, you may prevent the radiation but you won't be able to stop the fireball, vacuum suction, and pressure front from a nuclear detonation. You would have to introduce some Prothean technology that stops neutron emission. Then you run the problem of having enough power to sustain the population.

By politics, I meant the other races politics. That prevents the conduct of war. It only takes one side to start fighting. If there's no political will to fight, no government will conduct war and the Melchizadeck are left alone.

Look, the race can work, what I have issue with is the mary-sue-ness of the species. Overpowered weaponry, special "anti-nuke" technology, guardian-angel perception, suddenly being able to build ships and weapons without explaining how they got the infrastructure or knowledge.

*sigh* You're overpowering them and that will upset the balance of the setting since everything will have to deal with them.