The first video is running on Halo 2 Xbox, which has different restrictions then Halo 2 Vista. You could mix and match the campaign & multiplayer tags (assets) with more ease in H2x.
The second video is using AI that is already on a default campaign map. Each campaign map has the AI tags for whichever characters are in that scenario (that map or part of the story). Some maps have characters that are only used in cinematic form, via modding we can add them to the gameplay and and change the permutations of each character as well, so a Marine can be reconfigured into an ODST, or a hybrid of both.
We can reorganize the AI and change all of their properties but we cannot mix and match the assets from one map to another until we have the extracted models and animations for each character, or with a tool that could inject from one map to another. I have been asking around again for access to a tool that would allow the extraction of these assets, however, those that I have asked so far do not have access to it.