Many people wonder how the Myrmidon is supposed to replace the Ulysses. But if you read the tech descriptions and think for for a while, it makes sense. The Ulysses has great manoeuvrability and speed, but it better suited for offensive missions. And in the Reconstruction period, between the Hades Rebellion and NTF Rebellion, we have 30 years of relative peace and lack of assault missions. The GTVA was in need of a fighter that would be more suited to this defensive style.
Besides, they needed a jack-of-all-trades. The Herc II packs a bigger punch, Valkyrie (later Perseus, but it only went into service at the end of the NTF rebellion) is a much better interceptor while the Ulysses can fly circles around its enemies. The problem here is that usually those fighters would have to cooperate in most operations. So a small depot raid would involve for example a wing of Hercs for the destruction and a wing of Ulysses for protection. This makes sense during war, but in peacetimes it's just not economical.
A wing of Myrmidons, on the other hands, can do this job on its own. It packs enough weaponry to be a threat even for light capships like cruisers or freighters, can handle itself better in a dogfight than the Herc and still has enough armour to survive long enough for the reinforcements to come should something unexpected happen. It was built so that it could respond to to any situation, at least in a limited fashion. That's why it has three banks, to allow for a bigger variety of secondaries. A true multirole fighter. And remember that Myrmidon entered service (possibly a long time) before the Perseus.
You could probably ask why not keep the old Apollos around if they were better at multiroling ? Well, those fighters were already considered old during the Great War. 30 years later, they were probably seen as relics. The maintenance cost of them plus all the refits so that modern systems could be installed on it would prolly cost more than a brand new Myrmidon.
And don't forget one more thing - the Myrm was created as a Terran-Vasudan fighter. This probably helped improve the relations between Terrans and Vasudans too.