Great story so far , but into the codex , you only focused on the Frigates ,and Corvettes , What about Destroyers , Cruisers , Heavy Cruisers , Battleships , Dreadnoughts ,Carriers . ? Cant hardly wait for the next chapters .

The Citadel Codex can indeed be complex, but there are good ME wikis out there. There are also good Halo Wikis.
What hornet07 wrote is just a mix of obvious fanboyism and wild assumptions. Take his statement about mass accelerators: The official ME Wiki states that an 800m spinal gun has a force of 38 kt. And that they run about 90% of the lentgh of the ship. And here it gets tricky. a 1000m Drednought would have a 900m spinal gun. Power? Greater than 38kt, but we don't know how much exactly. When you assume a linear increase, you get 12,5 % more power, read 42,75 kt. But it's not stated in any canon source. Geth Dreadnoughts are stated to be 1100m long, they would have 990m long spinal guns.
The Destiny Ascension is again bigger, and has to my knowledge 4 spinal guns of unknown power. It's about 2km long, so you would have 1,8km long guns with 85,5 kt power. But, this is again not official. But not wild speculation either, since it uses mathematics instead of wild assumptions.
Vehicles: In the games we see the Mako, and another heavy duty vehicle as scenery prop. Sometimes it comes with a gun, sometimes it seems to be a cargo hauler. No name or other info is given. It is extremely unlikely that there are only two types of Alliance vehicles, but we don't know about the others. About the ground vehicles of other races we know simply nothing. ME just does not put heavy emphasis on vehicle combat. It is an RPG, Halo is a FPS. Mind the difference. Since you use some post Halo non-canon UNSC anyways, you should have no trouble to fill in the things you need.
Point defense lasers: We do not know how many projectiles they can shoot down. Bigger ships have more lasers than smaller ones of course. They overheat eventually when they fire continuously, but again, we do not know how fast. Salarians also have lasers that are much more powerful than those of anyone else. How much? We don't know, again.
Disruptor torpedoes: Not spammed by the hundreds, BUT: When they get close they power up their ME fields and become super massive objects that are nigh impossible to destroy, to counter point defense. That is official codex info, we are lucky here.
On ME armor we know very little. ME2 features Silaris armor as an optional, but expensive upgrade. Silaris armor is made of super compressed (courtesy of cleverly used ME fields during production) carbon nanotubes. That's definitely leagues better that some plain old titanium-steel composite -that's Titanium A, look it up on the Halo nation wiki. But Silaris armor is not applied in thick layers. About other ship armor: We simply don't know what it's made of, nor how thick it is. Something inferior to Silaris armor. Thickness? We can assume not more than 40-50 cm on frigates, depending on type, and 1m-2m on dreadnoughts, with cruisers somwhere in between. But we don't KNOW.
Kinetic barriers differ in strength, Asari barriers are the strongest though. They are triggered by anything with mass, that includes plasma and particle beams (made of PARTICLES, thus they're matter). Lasers are pure electromagnetic energy with no mass and bypass them. The energy projector should bypass them as well if the name is anything to go by (if it isn't a particle beam-I do not know, but maybe it is stated in some Halo wiki what an energy projector actually is).
When it comes to alien cultures the ME codex as well as the games give us a lot more info than Halo does. In Halo we only learn something about the Forerunners and the Sangheili, though not very much. Some of this stuff is covered in Halo novels, though they aren't canon. This is again due to the genre difference between RPG and shooter.
Bottom line: This a crossover and thus covers things that aren't stated in canon. You have to fill in many things by yourself.

frigates hold 1000 personal. most of the personal is marine so your specs are wrong. :(

A bit abrupt but I know how that goes with your first fan fic. Good setup thought, I'm looking forward to reading this one!

keep it up :)

Interesting story so far.

Keep it up. Laters.

Hey make sure, you get your names, dates and stuff correct.
People here are cruel, really cruel

It's actually a very short chapter. A few ships jump in through a relay, the home forces notice and charge up their defenses. That's the whole chapter. Forget the Codex if it means there is no story in your chapter.

Are you going to include the Absolute Record & Janus Key storyline into this story?

You sir have gotten my attention. Keep up the good work.

What about Strident-heavy and more?

Don't be afraid of the Citadel codex. After all, they all pretty much have the same configurations lol. That's just how stagnant and unimaginative they are.

Dreadnoughts:
- 800 - 1000 meters long
- weak armor as they focus mostly on Barriers

Armament
- a Mass Accelerator Cannon (38 KT)
- some short range PDF lasers that overheat and get overwhelmed by just a few dozen missiles (UNSC regularly fires hundreds from EACH ship)
- a few Disruptor Torpedoes that are never going to reach their targets because they're very slow and the UNSC is used to defending against much greater numbers

Comeplement
- probably a few fighters and bombers
- run-of-the-mill ground vehicles that come in ONE design only (like the MAKO of the Systems Alliance), because why would they come up with more diverse vehicles?
- as much of a troop complement as a dreadnought allows (invent the carrier dumbasses)

And that's it. Just scale down for the cruiser and frigates and that's all the Citadel has, except different paintjobs and art styles from the different species. Seriously though, the Citadel species really is downright incompetent when it comes to inventing, which isn't surprising since they just scavenge technology of the protheans and haven't advanced AT ALL in all their history.

The UNSC could crush their military in weeks if it came to a war. As for a Cold War, let's see them try getting information when everything is protected by AIs lightyears beyond everything they know.

Don't you dare try to nerf the UNSC or buff the Citadel. Show them like they are.

Interesting now regarding the coming conflict it depends on which citadel species went through the relay. You didn't clarify that much though I suspect it were the asari by the ship's design.
If that's the case then they would push for peace and diplomacy unless they are attacked first. The UNSC will proceed in a similar way but will be ready to press the trigger at a moments notice.
If the Turians went through the relay, well it depends how you want to portray them, arrogant and hardheaded or smart and reasonable. I'll need to know that before telling you how that could turn out.
If they are Salarians then they would go stealthly, gather info, return to citadel space and then the council decides how to proceed based on that.
If its other council race the like the turiams I need to know which species is.
So pm if you want.
Besides that good start so far, lets see how this turn out.

Interesting.

I was wondering is first contact going to be peaceful or is it going to turn into a planetside war? And also will the council be stupid as usual or will they be actually smart this time around?