Orange chromides don't need much salt to do well*; SG 1.002 at 25 C (5 g/litre) will be ample, and that's low enough you can keep them with all sorts of stuff. I'd opt for mostly medium-sized midwater species that aren't going to be harassed by them. Glassfish would be a good choice. Alternatively, provided the salinity was kept low as stated above, ticto barbs would be a colourful choice, as would virtually any of the hardier rainbowfish. Mollies should work, too. I wouldn't choose monos because monos need a big tank with fairly high salinity, and that wouldn't be easy to do with plants, and orange chromides will do best in a planted aquarium. Catfish and loaches aren't usually kept in brackish systems, but at SG 1.002 at 25 C, there are a good number of choices including brown hoplos and horseface loaches (the later need sand for digging though). Many of the spiny eels would work, too.
As for aquarium size, depends if you're keeping a pair or a singleton. Pairs are territorial, and obviously two males will fight. The species is not obviously sexually dimorphic, especially not in the case of the bright orange artificial form (the wild type is greenish with orange spots, turning black when breeding -- I think a more interesting looking animal, if not quite so colourful). Males do tend to be slightly more colourful when breeding, but the farmed specimens are so variable it's hard to tell a plain male from an unusually colourful female. In any event, a singleton or mated pair would be okay in, say, 75 litres, while you could keep a small group in something 150 litres or larger. By cichlid standards they aren't particularly aggressive, about on par with kribs.
Cheers, Neale
*Heiko Bleher has
videoed at least one population living in freshwater its entire life, but the aquarium specimens at least are sensitive to water quality and chemistry issues, and slightly brackish water does seem to keep them healthier.
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