I think it would be a good idea to have a 'Credits' page on the end of your story and have entries like this:
[ship/weapon/something name] concept - [ship author], hard-light.net
for example. And yes, I think it whould suffice to do this just once. Many bigger FSO mods stuff their credits into, well, credits section. Eventually these are also mentioned right in the end of the mods campaign (in a cutscene).
There was some discussion about licensing modded assets and mods some time ago and a shoutout to the community to start including licenses with their mods, so some might have them, but I don't think there's more to it than "Just credit me if you use this (for non-commercial purposes)".