jorgeDS: The Harem genre of the Main Story had been already hinted since start of the series, as the Author had since start (when writing the Web Novel) wanted this series to be a Harem and Chuuni styled story.
As for this spin-off adaptation, i don't think there is precedence of a Harem in it, but can't really be sure since its based on an original Light Novel adaptation that the Author is writing currently (so except some minor stuff on what happened in the past, there isn't much known about it).
By the way, if i go by the Light Novel version comparison between this series and the Main Series, then this series has more epic scale battles and dark theme as well.
Also, there are a lot of things which hints to the events that occur in the Main Stories, especially one key factor's origin is showcased in this adaptation (chapter 8, to be exact), which directly helped the MC in the Main Series.