There is a small green alien in a film.
There is also a tall blue alien in the film.
You write a book where green aliens are offshoot of blue aliens and you make the green aliens tall as the blue ones in your book.
Would you say, that there is an evidence (or proof in other words) in that original film that green aliens are as tall as the blue ones?
Of course I would. I have just co-opted somebody else's fictional realm and spun a story out of that, with the specific purpose of exploiting the elements in that realm. I now dictate the rules, and one of them is that the blues and the greens are related (in the sense of being the exact same species, just living in different planets). Under the new rules, small greens and large blues are indirect evidence of the existence of large greens and small blues. Not under the old rules, though - but I just outdated those.
Foster and FASA make self-consistent cases under their own rules, and can pervert any onscreen bits to their advantage there.
(This is essentially what the writers of "Homefront" did with the material on Joe Sisko that they got from "The Alternate", an episode where it was assumed that Joe was long dead... They took the bits they liked and used them as "evidence" that would support the latter episode's take of Joe as a fighting man who had defeated an illness. The writers of "The Alternate" just didn't write airtight rules on Joe Sisko, allowing the second set to exploit a loophole.)
You'll drive yourself insane. Just choose a height your Caitian needs to be.
Exactly. Your story, your rules. Although if you want, you can always use somebody else's rules as well. Or exploit the inevitable loopholes in them.