Does it sound stupid that a SPARTAN-II is captain of a UNSC Frigate? (I made the story up)
Not really, it sounds pretty cool.
Serin Osman says “no”. Although she was technically a Spartan washout. Grey Team has control over their own private ship though.
Depends what the Frigate does. I wouldn’t say it’s stupid, just canonically it doesn’t make that much sense unless it is a spec ops frigate or something. But it’s great for fan fiction. There is a Spartan team with their own ship, it’s like a modified freighter or something.
Grey Team had a freighter but they were given a Prowler later on.
To make it more interesting you could make it that he was one of the s-IIs that didn’t well through augmentations and could have some sort of disability just my train of thought.
If it was a fully functioning and combat ready Spartan II, then yes, it’d be odd.
But if it was a washout, revived or somewhat recovered by ONI, it can work. Their minds would be enhanced, rhey could make great tactitions and strategists.