I don't know very much about cichlids, can you name any species in particular that would not cause problems?
To do this you must use the 125, there is no room in the 55 for them in your current setup. You must decorate wisely, taking into consideration the needs of the cichlid species you choose, and you may have to make some modifications to your current stock to make room. For example, most of your species are bottom dwellers that enjoy cover, as do the larger number of cichlids - this would result in unwanted aggression in disputes over territory.
My biggest concern is right now is your Leporinus, known for becoming hostile as they mature, and as fish that like cover they will likely compete with most cichlids for territory, and probably win in many cases. It's better to plan the tank around the cichlids rather then the other way around, but some ideas are possible - keeping in mind that when it comes to cichlids there are rarely guarantees of success, the best we can do is arrange it for the greatest odds.
That little blurb wasn't to dissuade you from the idea, but merely to keep you aware of the con's. The pro's are unique and beautiful fish with unique and intelligent seeming behavior that never gets tiring. Click below to see my successful cichlid/community mix. The cichlids you see are Nicaraguensis - one idea that could likely work without the Leporinus involved. Should you still be interested in the cichlid additions let me know and we'll dig up some more species for you to consider. One possibility that won't get in the way of your bottom dwellers is the popular Heros Severus.
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