mostly it depends what SDL supports i'd say. you can always convert it to flv, it may be outdated but it still exists.
What software is recommended to convert videos to FLC with audio (like the ones from TFTD, i'm guessing FLX)?
I tried using RAD Video Tools but it only converts to FLC without audio, and when tested ingame the frame rate is about 2x faster than the source. When I play the FLC through QuickTime, the video plays with the proper frame rate. I think the same happened on this thread:
https://openxcom.org/forum/index.php/topic,3639.0.htmlI also tried converting to FLC/FLX through Autodesk Animator Studio and Ulead Video Studio on a legacy virtual machine, but apparently they encode the chunks in a way that isn't recognized by the video player, because the videos are skipped as soon as they are loaded. I believe the original videos were recorded using EGI.
As for the intro, right now it seems that only FLI is supported, because you need to rename your file exactly to UFOINT.FLI if you want to override it (if you specify another video in the ruleset, it is played after the original intro because it only adds it to the video array), and if you just rename the extension of a FLC file the game crashes.