In a 40 gallon, you would be fine with a pair of these and some dithers. small school of tetras would work well, maybe some cories. These guys are from Costa Rica, if you wanted to do a biotope, you could keep them with some wild variant livebearers.I will have to look further in to these as well, although I'm really liking the sound of more of a community tank and it might be crowded with a pair of these and dithers of some sort...
That said, the west African thing would be cool too. I'd go with whatever you like more. But both are plausible for the tank size.