The Myrmidon is a more stereotypical space superiority fighter than the Ulysses. I have always treated the latter as an interceptor. I don't know how a fighter of its size and statistics could be called anything else.
The distinction between air superiority fighters and interceptors did not really have anything to do with size. One was agile and optimised for dogfights, the other for beyond visual range engagements with an emphasis on missile armament and raw speed. The Ulysses is definitely high on agility and, while fast eough (faster average speed than a Perseus on full power to the engines), doesn't carry an impressive missile load. Definitely a space superiority fighter.
About the Myrmidon, if it's not a Space Superiority fighter, what should it have been called? Medium Fighter? Space Superiority bomber?
Thematically, Freespace ships are a mishmash of cold-war-era aviation garnished with ww2-era dogfighting. I'm not sure whether this is intentional, but more than any given type the Myrmidon feels very American. Large, complex, fast, well-armed and expected to ensure space superiority against less powerful but more nimble foes.
Space Superiority Fighter? Maybe, see above.
Interceptor? Speed is barely adequate and there's more emphasis on gun firepower, but this could work.
Assault Fighter? Very flimsy for this role, firepower isn't entirely convincing either but it's faster. Strike fighter?
I think it's a true multi-role craft... usually an awkward one but it has its moments.
Also, in the Freespace universe 'advanced' seems to mean something strange because anything with that word in the description is deeply flawed for what it's supposed to do.
The 'advanced space superiority fighter' Myrmidon is sluggish and can't mount the iconic missile (Harpoon).
The 'advanced interceptor' Serapis is slow and can't mount the iconic missile (Trebuchet).
The 'advanced bomber' Bakha is just not very good compared to the Sekhmet and can't mount the iconic missile (Helios).