I've been feeding my girls Marine-S Hikkari pellets, they soften up pretty quick and even the tiny little girl I got tonight was able to gnaw through one.
Most of the food choice depends on what you can get locally. I've used Hikkari Marine-S, Fancy Guppy and Micro Wafers. Of all the different freeze dried blood worms I prefer the Omega One but I avoid their flakes like the plague.
Easiest way to determine sex on a young betta is to startle it. Males will dart around and females will hide and show stress lines. The males cannot show stress lines tho they WILL usually display a youth stripe down their side that can look like one.
The regular little feedings and temperature of the water as well as the cleanliness of the handling are very important for the young ones. If you stop seeing fresh poo in the bowl, stop feeding and watch them closely, sometimes they grow quick enough that their guts simply don't need to dump as often.
The smallest girl I ever brought home was Emerald, she was also the only one that stayed green, at 18mm lip to fin tip. She had a visible ovipositor.