The con-specific thing isn't an unbreakable rule. For one thing, it's conditional to there actually being con-specifics in the tank, no con-specifics and you can throw it out as predicting their behavior, then it just depends on the individual fish and what else is in the tank. There are other factors that are conditional; for example, a breeding pair may try to chase fish away from a spot they like for spawning-- even when they're not spawning.
Overall I agree with the points above. It's definitely possible to keep them as a pair with other fish, I've done it, but yours is apparently expressing whatever frustration or aggression or loneliness to the other fish (whatever loneliness means in fish terms, in this case I simply mean your geo may prefer to see other geos in the tank).
...Sounds like your severums are same size as the geo or are they a bit smaller? IME redheads geos and sevs are a good combination and, if anything, the sevs would be the more dominant fish, given their larger adult size.