Fly larva with the exception of one species all will only eat decaying meat. Interesting enough that they have found their place in the medical community now. Since they only eat dead meat they are being used to treat people with open sores that will not heal. The eat off all the bad parts and their saliva is so acidic that it will close the rest of the wound and promote good growth back.
The flies themselves should eat about anything. Post a link to the site or if you have a specific scientific name. Someone here probably has used them or tried them, perhaps that will help you get more specifics form someone with experience with them.
I gutload the flies after they hatch for about 24 hours before feeding them to the chameleons with a mixture of water, powdered buttermilk, a small amount of powdered sugar, some powdered alfalfa pellets, and a handful of WER gutload. My aim is not to rear healthy flies.. it is to provide a healthy meal to my chameleons. Because of that, I don't feed rotten meat or dog food to the flies.
You don't need to do anything to your larvae for them to hatch. They don't need to eat. Just keep them warm and they will pupate. You may need to open the lid a little to keep the moisture content down a bit. The only thing you need to worry about is getting them gutloaded before you feed them to your chameleons.. and that isn't even necessary, just healthier. When I first started getting them, I just opened the container after they were flies and let a few fly out. Or, froze them for a few minutes to slow them down and then sprinkled a few out. Be prepared.. flies are a fantastic feeder, but they get loose in your house pretty easily. Be careful how many you feed at a time.. only enough that your chameleon can take care of. Also, you might want to stagger the larvae hatching by keeping them cool. I wait for the larvae to pupate and then refrigerate all except for a container of them.
I am sure that others will have some tips on keeping flies..
The ones from grubco (blue bottle) are about twice the size of a house fly - maybe a tad bit smaller. Every so often there is a few really big and a few really small flies in the batch. Most are too big for pygmy chams.