Looking at your other
thread, you've tried a blood parrot (severum bullied it) and a convict (bullied the severum).
Even with moving the decor round I would be reluctant to try another cichlid as those are what the severum has problems with. Maybe try again if you get that bigger tank you mention in the other thread.
I've just looked again at Seriously Fish and in particular its tank size requirements. I now think the reason you are having problems with the severum and other cichlids is tank size. SF recommends
Its not really suitable for tanks less than 48″ x 18″ x 18″ (120cm x 45cm x 45cm) – 243 litres in size.
You currently have a 55 gall tank. If that's American galls, it's 208 litres; if it's Imperial galls, that's 250 litres.
The sev is behaving like this because it is stressed as its tank isn't big enough. Even if your galls are Imperial, the tank is the minimum size the sev needs. Fish often behave against type when they are stressed, becoming either more aggressive or more timid.
I would not get any more cichlids until you have upgraded to the 100 galls you mention in your other thread. But you do have room for non-cichlids. Using the Community Creator and 209 litres (ie 55 US galls), you have a lot of 'unused' space. In the fish profiles, rainbow shark is called ruby shark and skirt tetras are black widow tetras (just different colours of Gymnocorymbus ternetzi).
[I used American galls in the CC as they are more common in fish keeping].
Until you upgrade, personally I'd get more white skirts (white widows) and maybe 10 to 12 of another shoaling species. But I would wait until after the upgrade before trying more cichlids.
PS - are you located in the USA? I just wonder as you give tank sizes in galls rather than litres and you call your tetras white skirts rather than white widows.