Very small juveniles often don't fare well. The same holds true with mature adults, which often don't adapt well to captive life. I've kept different bf's over the years. If you find one in the 2-3 inch range that looks healthy and is eating, it's usually a safe bet. With butterflies, there are a lot of variables, and certain species aren't good captive choices, so make sure the sp. you plan to keep has a relatively good track record.