The rules are complicated[0] and they say "If all valid claims monetize the video, revenue is divided by the number of claims except in special cases such as cover revshare and music."
So just having content from multiple copyright holders doesn't necessarily stop ads from playing. However, that policy also says "If one of the assets claiming a video has missing ownership information, the default policy action is Track (owner missing)." which "Allows video to be viewable on YouTube and tracks viewership, but does not serve ads against it."[1]
An alternative might just be to add some content at the end of the video which is not advertiser-friendly, which would demonetise the video while not annoying the viewers too much.
So just having content from multiple copyright holders doesn't necessarily stop ads from playing. However, that policy also says "If one of the assets claiming a video has missing ownership information, the default policy action is Track (owner missing)." which "Allows video to be viewable on YouTube and tracks viewership, but does not serve ads against it."[1]
An alternative might just be to add some content at the end of the video which is not advertiser-friendly, which would demonetise the video while not annoying the viewers too much.
[0] https://support.google.com/youtube/answer/6300781?hl=en
[1] https://support.google.com/youtube/answer/6085494?hl=en