There is no hard rule other than to offer appropriately sized prey. This premise is open to interpretation though as one frog may more eagerly take larger prey than another. Personally I’d wait until you have a snout to vent length of 4 inches before offering small pinkies. Just because a froglet can take a pinky it’s own size doesn’t mean it should. If anything, once the time comes that your frog is large enough to take them I’d offer pinkies only as an occasional supplement. There are varied opinions on mammalian vs insect feeder sources. Unfortunately there are conflicting and questionable nutritional contents posted online. It’s difficult to get an accurate idea of how nutritious mice are in relation to frogs needs. Offering a few pinkies here and there is harmless as long as they don’t compose the bulk of its diet. Also I don’t see a need to supplement pinkies with calcium. Their bones contain enough. Supplement insects with calcium and vitamins at regular insect feedings. As an aside I can also tell from personal experience that frogs fed exclusively on mice live shorter lifespans and are prone to obesity.