The first time I tried replacing a Bink 2(.bk2) video with a Bink 1 video(.bik), the video played, started to glitch and then GalCiv 3 crashed. It might have been due to an incorrect resolution. I can't remember.
Just to see if anythings changed or I was doing it wrong. I just tried replacing the Galaxy Spinning video by converting the .bk2 to .bik using the official RAD Game Tools. Then I renamed it to .bk2 as GalCiv 3 will skip files ending with .bik. It worked. No crashes in either new game screen or the loading screen where this video also used. Here is a screenshot:
Notice the black background for the Galaxy in above screenshot
However, while Bink 1 supports an alpha channel for transparent pixels, it heavily glitches if the setting "alpha plane options" is any other than "Do not include the alpha plane". Once it finishes a loop, the glitch clears and starts glitching again. As seen here:
I then created a Bink 1 video with plain Red, Green and Blue images and it worked.
I even tried converting a whole 1080p video. That also worked. Though there still may be an unseen performance hit.
RAD Game Tools downloads page - http://www.radgametools.com/bnkdown.htm