i think most of the destroyers in FS2 are around 1300 or so poly's so you should be ok, to make LODs you have to name the object that's gonna be the LOD detaila for the first level after the close up one ( you want it fairly similar to main one cos this is the one that appears in your target window ) and then detailb, detailc for the other two.
Now the question is, are you using Truspace?, because I know to get the model to work you have to have the bits and pieces in the right order or hierachy you can find out your current hierachy in truspace ( at least in truespace 2 anyway ) by looking at the animation properties box. You want the order going down the screen to look something like this:
(shields go here)
don't panic if the're not in order, it's easy to correct, simply save each individual bit as a seperate object, then open a new scene an load them up in the right order starting with the hull. ( That last bit took me a few months to think of, and it was a right hassle sorting it out I tell you - just proves simple ideas aren't always the first you think of. )
You then have to open the saved object/scene in a hex editor and do a ANSI search for detaila and change it to detail1 - do this for the others then save the changes.
Then hopefully when COB2POF runs it should make them LODs - you can easily check using segultech
Note, that you might be able to name them detail1 etc, straight away if you've got truspace 4, to check, once you renamed select a different object and then go back if it's now called detail,1 you will have to call it detaila and do the hex editing, which isn't that difficult really, just remember to make lots of copies of your work before you do anything you're not sure about.
oh, and then of course you have to specify the LODs in the ships table entry under detail distance simply copy those form another ship which is of a similar size.
I think my fingers are about to drop off...