So I thought for completeness's sake I'd update the status on them in case someone's interested in dispar longevity and hardiness in the future.
It's been 10 days since 4 small dispar arrived in the mail. Looking closely, there has always been a distinguishable difference between all of them - one big, one a little smaller, one smaller still, and one that looks hungry!
Over the weekend, the one that looked hungry died - he had never really been seriously feeding. Sometimes he'd swallow a cyclops, and he'd eat a few BBS when I offered them, but he never got with the program.
The next-bigger fish (#3 of 4) looks skinny also but does eat an occasional fish egg and BBS. I don't have high hopes for him but perhaps he'll make it.
The biggest two love to eat and are rapidly putting on weight. Now I'm just wondering whether it's a mistake to have just 2 dispar in a tank. Will the big one turn into a male and beat the littler one?