I want to find and kill the guardians ASAP so that I can play around with their weapons during my playthrough(rather than leaving them until the end when it would be easy with a large, pimped out fleet but then there would be nothing left to do anyway).
What would be good early accessible fleet compositions for this?
As it happens that was the premise of my last playthrough. Speedrun the hypershunt battles, and then savescum them to try to get the weapons I wanted. (Result: the types of weapons dropped after the battle is largely fixed, however there is some randomness to the quantity of each weapon type dropped. So if you want multiple copies of a specific weapon, then savescumming can be worthwhile.)
The danger posed by Omega's is twofold: high damage output of the Tesseracts, and swarming effect of the shards and fighters. To withstand the high damage output you want to use capital ships, and possibly Apogees and Monitors. Less tanky ships than those will get picked off and destroyed too easily. To deal with the swarming enemies I find Locust missiles to be particularly effective.
Getting an Alpha core Radiant is the single best thing you can do to prepare for these battles. You can reliably get an alpha core from the scientist's tech cache mission, and you can get an easy guaranteed Radiant encounter from the Red Planet mission (although you may have to savescum the fight for the Radiant to be recoverable). The best possible Radiant loadout in my experience is 3x Plasma Cannon + 2x Locusts + 4x Sabots, with some burst PD in the rear.
The other key ship to look at is the Conquest. Of all the capital ships the Conquest is by far the easiest to get, by buying it from the black market on Neutral, Diktat or League planets. It isn't the tankiest ship out there, however it brings a lot of firepower to the table including two important Locust mounts. Use a symmetrical weapon loadout with Locusts. For example, Mark IX + Mjolnir + Mauler + Dual Flak + Ion Beam on each side.
My fleet for the omega battles ended up being an alpha core Radiant, two Conquests with lvl 5 Officers, and player-piloted Paragon. All with 2 S-mods. Altogether I did close to 20 omega battles and won easily every time, only once or twice losing a ship. The strategy was ordering the Radiant + 1 Conquest to Engage one Tesseract, while the other Conquest and myself in the Paragon dealt with the second Tesseract. My overall impression is that this fight is actually easier than endgame Remnant fights. For one, I am honestly more scared of alpha core Radiants than I am of Tesseracts. For another, the absence of a dozen frigates and destroyers constantly flanking your ships helps a lot. Having just two enemies to worry about makes everything a lot easier, particularly for the AI, even if they are tough enemies.
If you don't have a Paragon (I was lucky to have one), then I recommend piloting a Monitor to draw fire from one Tesseract while ~3 Conquests are shooting at it. So a fleet of Radiant + 4 Conquests + Monitor should do the trick.
Finally, do you need to be lvl 15 to do it? Well you don't need a lot of personal skills if you sit in a Monitor. You need to be lvl 10 to get the Red Planet mission though. Also reaching lvl 15 is really not that difficult if you optimize for it. Spend your story points on 100% bonus XP options to have constant double XP, and maximize your XP bonus by using a small fleet without officers. If you are a decent pilot you can solo a lot of battles. Early game with SO Hammerhead or SO Falcon, later on with a Conquest, Odyssey, SO Eagle, SO Aurora or Doom. Or indeed the Ziggurat which you can get extremely early if you know where to look, although I'm not a phase kind of person so I wouldn't know.
