You can always PM the listed authors (assuming they are still Hard-Light members)
Failing any communication, then in the Credits, you would list:
"Ship X by ThisGuy as shown here: WikiLink, Downloaded from: FSMods/Wherever"
And that should probably cover you pretty well. But do try to take the approach of reaching out as best you can, if all else fails, do a follow up post here where you specifically outline any ships you haven't heard from the author on.
That is, if you're for being super polite about it. In general, if it's publicly available, it is usually publicly usable. (Unless stated otherwise, I guess, but that's never made any sense to me)