I've got a group of Fundulopanchax gardneri gardneri 'Nsukka Gold' in my main tank with 43 cardinal tetras and they do great.
In another planted tank I've got a breeding group of Epiplatys dageti 'Nkwanta gh96' with pencilfish and breeding Apisotgramma eremnopyge.
With cardinals I'd go for something that's going to stay nearer the top so Epiplatys would be an excellent choice. There are loads of species, have a look at Tim Addis's Epiplatys web page. I have seen a number for sale on Aquabid Killifish eggs page.
Hope that helps!
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