I'm unsure how you got that the fadeout action was only usable with playlists. The <<audio>> macro documentation clearly lists it as an available action.
Regardless. To fade out all currently playing audio, which hasn't been copied into a playlist, and also start a new track playing looped, you want something like the following:
<<audio ":playing" fadeout>><<audio "track that is not playing" loop play>>
Also. Do not confuse playlists, created with the <<createplaylist>> macro, with either the built-in groups or custom groups, created with the <<createaudiogroup>> macro. A playlist is a queue of tracks to play in some sequence, controlled via the <<playlist>> macro. A group is simply a list of tracks which allow you to issue commands to multiple tracks simultaneously, controlled via the <<audio>> macro.