D'oh! I had originally mentioned something, and then must have deleted it while I was revising my comment: The B picture came first. So technically, Star Trek would've been in the right place.
As for Rocky: Good question. Fox made Rocky as an A-movie, but then deemed it unreleasable in the U.S. (I bet there's a good Wikipedia entry on this, but I'm too lazy to link it). It did well everywhere else, but by the time Fox did start showing it, they had relegated it to B-movie status (though I don't think they officially considered it one).
What would Rocky be paired with? Dunno. Even if you were to confine your choices to movies made during the same time period, you'd have lots of choices as far as B-movie spoofs and crappy musicals are concerned. Personally, my vote would be for Shock Treatment
, the little-known sequel (almost) to Rocky Horror.
Briefly, Shock Treatment was originally conceived to be a sequel, but Fox decided to sever Shock Treatment's conceptual ties to the previous movie because Rocky wasn't widely known and most of the principal actors were unavailable. Richard O'Brien heavily
revised his script, the movie was made, and it was billed as being "from the minds that brought you Rocky Horror" instead of being "RHPS2: With a Vengeance." Is it any good? Well, there's a reason it's not more popular, and I hate mentioning it to people who've never heard of it because I don't want to get their hopes up. But the songs are
catchy, and once you get past the initial "wait! This isn't what I expected!", it's a fun movie.
Incidentally, O'Brien did write an actual, genuine sequel in the late '80s which he sold to Fox, and which is unlikely ever to be made. The script is available online, and when I first encountered it I mistook it for fanfic, but Mr. O'Brien confirms that its the real thing. If it sounds interesting, I recommend it. It's called Revenge of the Old Queen.