About tank aggression?? Please advise

[Gourami Swami]

I would say if you re-home the synspilum, some decent suggestions would be
Severum
chocolate cichlid
few more firemouths
blue acara or two
guianacara gaeyi

I wouldn't do all of them, tank is pretty full already, pick your favorite and replace the syn with it. These fish won't be as aggressive, will probably live peacefully with your Oscar.

 

[duanes]

Due to some DNA research, it was found that synspillum, is a color variant of melanurum, so same species.
The reason melanurum was chosen as the valid name is that the first scientific description of melanurum was in 1862 by Gunther, the name synspillum wasn't used until 1935 (Hubbs).
The name used in the first description, trumps the later, hence melanurum is proper, although many aquarists still like to use synspillum.
I believe the tank will be overstocked even if you remove some fish, some in the tank are not full grown, so to me adding more, is not a proper option.