I played around with NDF and by cutting/pasting and some guesswork I got a modified version to work.
First, I built a new map using the map editor- the hexes flip normally, planets/bases/shipyards show up as they should, all is well.
Then I cut the Cardassian ship section out of DefaultCore and DefaultLoadOut from NDF and pasted it into the stock SFC3 with the v534 patch or whatever the latest was. I found the file "Invalid Ships had almost all the Cardassians overloaded, and when I played the game I only found freighter convoys...every other ship came up as "Defender has already died" when I tried to attack. I read somewhere you could modify an overloaded ship in the refit screen, save it as a variant, then replace the stock model with the variant...anyway, I added a Cardassian frigate to the Federation, but it didn't show up; I compared the metrics to the Saber, and modified them so they were closer in size- sure enough, it showed up! I was able to buy it and fly it, no problem.
Turns out the Cardassian ships are about 10% the size of the stock ships with regards to shield space and power space. Adding a zero to these numbers enabled me to successfully load all the remaining Cardassian and Dominion ships...it was actually kinda funny, cuz at first I had nothing bigger than a light cruiser so I'd get bounced by up to 5 at once...as I added more heavies, the numbers came down.
I think the numbers might be too high, so if someone wants to look into that I'd be much obliged. In the meantime, using the race swapping technique found in the online mod guide, one could easily substitute the Cardassian/Dominion for Klingons, Borg, etc.
If anyone wants more detailed info or a copy of my specs, PM me here...
*****I take no credit for any changes to the stock game or NDF. These are the fixes I had to come up with in order to make the mod work for me, and I happily share them with anyone having similar problems. If the owner of NDF would like to incorporate either my map or my DC/DLO files I'd be more than willing to share.