I'm thinking about taking the plunge into the whole SCP modding scene, and am willing to go through whatever it will take to make that stuff work, but I have a serious question at the outset: how many of the engine/models/nebula/etc. items out there will work in the original FS2 SP campaign? How many are set up to do so from the outset?

If the new nebula's in particular don't overwrite existing ones (i.e. you don't have to open each mission file in Fred, redo the entire background scene, redo the entire ship scene, etc., and then save all that), I'm probably not going to bother with this at all. User-made campaigns just have no appeal at all to me for some reason, so if that's all that present day modding applies too, I'd like to know before getting my hopes up.

The SCP media VPs replace much of the original campaign, i.e. new ship models, nebulas, weapons effects etc.

If you create a fighter with the same model file name as one already in the game FS2 will simply swap the new one for the original.

Capships are a little more tricky as there may be certain missions which expect certain numbered turrets to be present.

Pretty much any graphic file can be replaced so simply reskinning ships is quite possible. In fact that was the extent of Freespace modding at first.

Finally there's a whole bunch of work going on to make high poly versions of the original FS2 ships for use with FS2_Open. A lot of them have been done already but there are still quite a few left.