Sounds like you're on the right track. You have to keep what you like. As far as the Fstavum go there is about 5 or 6 I think it's 6 now different species of Mesonauta but they are all generally the same coloration nothing overly colorful. Severum are a great center piece by themselves so take your time and consider all the options carfully no need to rush.
I never heard anyone say they thought a chocolate looked like an oscar that's a first.
PS you can do a mixed school of corys.
I definitely will not rush it. The tank isn't even here yet. It will be several weeks after I get it to cycle it and have it ready for fish. Once it is, I will be housing the keyholes, rainbows and almost everything else except the severums in the tank for a week or two to let the keyholes settle in and own what they want. When that is done, I will add the severum to the tank and wait another week or 2 until everything settles down. So it will be a month or 2 at the earliest before I will be adding anything new to the tank that I don't already have.
Anchovie recommended angels and I like the idea since I have plans for an angel tank in the future. What I plan to do with the angels is have them breed a few times in their community tank, then remove them to a breeding 20 gallon high (I would have 2 running at all times) and remove them back to the community tank once they have bred. Think this could work with severums in the community tank? I would mainly worry about how the severums would react to the angels being removed and added back so frequently.
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