but I can't figure out if they've watched it/how they feel about it

That right there is enough reason not to do this. Absolutely not. If they're unrelated canons and not part of the same universe/franchise/whatever, do not do this.

There are specific situations where I might do an unrequested crossover of the actual fandoms the recip is requesting if their sign-up makes it clear that they're open to unexpected stuff like that... but only then.