I am unframilliar to severums as far as breeding and sexing is concerned however I am going over to a feinds place tommorow eve I will ask him as he know more about them than I. Hopefully I can get you an answer by tommorow nite
The only way that I would know at this size to have any chace of the sexing them would be to vent the fish. I don't know how easy it is though to sex severums through venting.
You can do a search on here for more info on venting, or google it.
yeah their dorsal fins will be different, if you have a male and a female next to each other, the female will have nice rounded fins and the male will have "pointier" (?) fins. especially the dorsal. so if theyre big enough to tell apart that way then kudos, but like they said youll probably need to wait til theyre bigger.