This post has a tag scoring sheet, copied over from another site, to determine what makes a good tag. Let's see how credits scores:
- Does it make sense to be an "expert in credits"?*
No; score -1.
- Does it make sense for a question to be tagged only with credits"?
No (there should also be a tag for the film or franchise in question); score -1.
- Does credits have a single, universally-unambiguous meaning?
Yes (as you defined in your question); score 2.
- Is credits likely to be used correctly just based on its name?
Maybe (although there are other meanings of the word, as mentioned in your question); score 1.
- Are there "enough" (> 15) but not "too many" (> 10% site-wide) questions that qualify for credits?
Maybe? Someone would have to count them to check, so for the moment I'll say score 1.
- Are people like to use credits to find questions to answer?
Maybe? I'd say probably not, but let's say score 1.
- Are there likely some users (be objective!) who will favorite or ignore credits?
Maybe? The fact that you've asked this question shows a certain level of interest! Score 1.
- Could credits be reasonably used to feed questions to a specialized chat room?
No; score -1.
- Can credits be used to search for questions (for any reason) in a way that keyword searching cannot accomplish?
Yes (because as you said, the word has more than one meaning); score 2.
Total is 5, which is apparently not enough for the tag to be worth creating. From the linked question:
tags that score > 12 are "good" tags, tags that score < 8 are "terrible" tags, others are likely good but may need some clarification/renaming/etc