Just an update -- it appears all seven fish are male.
I have gone back through the seller's feedback on Aquabid and emailed everyone else who bought D. foirni from him, and so far it sounds like most people ended up with all males. A couple think they have one female but aren't sure yet. I know water parameters probably affect the sex ratios of the fry to some degree, so maybe most of the ones sold were male. It's a real bummer to know that it's the end of the line for these fish unless I find a female somewhere.