I wouldn't put more than one adult gray treefrog in a 12"x12"x18" tank. I also wouldn't consider mixing another type of frog with it given that this sounds like your first frog and definitely not in an enclosure this small. Your frog has no need of a companion, they don't get lonely, they don't need friends, just food and an appropriate enclosure. You might consider getting some live plants to go in your tank. Pothos, aka Devils Ivy, is a readily available and very easy to care for plant if you want something simple to start with.

You can try earthworms as food as well as the moths. Earthworms are awesome nutritionally but not all frogs have an interest. Make sure any feeders you collect are from a pesticide and chemical free area.

I've never heard of the yellow/orange colours inside the legs being used to communicate any kind of warning to other frogs. They're referred to as "flash colours" and are believed to startle predators when the frog goes to flee by suddenly becoming colourful. Females have the same colouration as males here.

I don't find the throat being dark or light a reliable indication of male or female. I have 2 males and one female, one of the males has a noticeably lighter throat than the other two, the other male is pretty much the same as the female. I've also seen numerous wild calling grays with light throats. It could be my local population though.

You don't really have to worry about them breeding spontaneously in captivity if you did add another one. They generally need to be cycled (go through a cooling period) to breed and then kept in very warm and humid (or even raining) conditions and have an appropriate place to lay the eggs. I don't cycle mine and my female shows absolutely no interest in the advances of the male she's in with.