Other than being 'a great little cruiser,' Vonda McIntyre really doesn't say what class the Lydia Sutherland is, or what she looked like. The Franz Joseph Star Fleet Technical Manual, the first place the Saladin-class appears, clearly states it's a 'destroyer/scout' ship. The Saladin-class seems to be generally accepted as existing in the Prime Universe (thanks to popping up on monitors in TWOK/TSFS), and is definitely around in the right time period. So, if the nacelle fits...
Well, I'm wary of the "if it's on a monitor, it must be real" argument, because then there'd really have to be a giant rubber ducky somewhere on the
Enterprise-D, not to mention all the
Dirty Pair and
Buckaroo Banzai and other in-jokes in TNG/DS9/VGR graphics. And honestly I was never that big a fan of the FJ stuff. Although I guess the existence of the
Kelvin sets a canonical precedent for single-nacelled vessels.
The Miranda-class has always been a light cruiser,
I've seen tech reference calling it a "frigate," which Wikipedia says can overlap with anything from a corvette to a destroyer to a cruiser to a battleship.
and (while it's not impossible for Kirk to be flying an older ship near the end of service life) the Baton Rouge-class (which, from configuration alone, must be a heavy cruiser) really doesn't look like it'd hold up too well in a fight. So 'destroyer-equivalent' seems unlikely.
"Destroyer-equivalent" doesn't mean a combat vessel, it just means a smaller ship of the sort that would be commanded by a junior officer or novice captain. Although I agree the BR class seems to be larger than that; the references I can find make it nearly as large as a Connie.
Still, we don't actually know that the
Enterprise was Kirk's
second command. TMoST doesn't specify that. He could've commanded another ship between the "destroyer-equivalent" and the big E.
And why would it have the name Saladin if the Saladin-class is flying around?
Mike may have intended it to be an FJ Saladin-class ship, with artist David Ross choosing to go with a
Baton Rouge instead. Although maybe not, since an earlier Trek comic Mike wrote for Marvel depicted the
Republic, on which Kirk served as an ensign, as a BR-class ship.