My general rule is to feed every other day unless the mantids bum is still a little skinny. There are a few species that can be over fed. I believe orchids, theopropus elegans, pseudoharpax virescans are some of the ones who have problems with over feeding. With my small nymphs i put a couple ff's in for each (i keep in batches of 5 until 3rd instar), and for my bigger ones it depends one how big the mantis is, but it can range between 2 greenbottles to 5or 6 greenbottles. My really big species (viridis ect) i feed green bottles, blue bottles, moths, and locusts. When the bums look plump i leave them for a day or 2 to let the bum flatten down a little. A plump bum can be a sign that the mantis is due to moult too, so i dont generally feed until the mantis either moults or the bum looks a little skinnier.
I think a mantis will eat whatever it comes across, and i suppose this is due to the wild tendencies because they dont get food every day in the wild, so they have to eat when they can because they arent guaranteed to get a fly or 2 the next day.
Sorry for the essay!!!!
Tina x