I got 4 of these from Rapps a couple weeks ago that were 1.5". With obviously extremely limited experience I'd say you wont' have to wait all that long. They grow like weeds, and as far as food goes, mine act like miniature oscars. I walk near the tank and they come running to the front to tell me what a bad fish dad I am, and how they're starving to death. If you want a guess based on limited observation, I'd hazard a growth rate at this point of solid 1/2" (maybe a bit more) per month.
My understanding is males tend to be markedly larger than females, the pointed dorsal fin deal. Mine started at the same size, and already 2 are markedly larger. The larger ones seem to have dorsals that are a bit more pointed. The smaller two have the bar on the dorsal you mentioned. I think of them as pint sized personality plus.