When you can use an Ergo (or any SSC with heavy waist padding) without an infant insert depends more on the length of the child's leg rather than specifically any developmental milestone like the ability to hold their head unsupported. Likewise, weight recommendations are just a guide. The legs should be long enough to straddle comfortably without the sides of the carrier rubbing or pushing against the back of their leg, or without the legs being extended past a comfortable straddle. In my very tall son's case, this was at three months. For some babies this is at four or five months.

A word of caution about using most SSCs with an infant in froggy position. Although it can be done, and done safely, no manufacturer will recommend it. Because the waist is structured and cannot be worn apron style like most mei-tais (and some buckle tais without structured waist like the Connecta), which forms a pouch, potentially a baby can slip through the bottom of the carrier. Whilst it is highly unlikely that they will fall through unless the waist band is loosened accidentally, what is more likely that if the top of the waist band is not perfectly snug (not uncommon in a carrier that is worn on the hips), the baby's feet can become wedged between the band, potentially losing circulation. Now, most if not all babies will let you know long before this is a problem, and proper positioning should be checked using any carrier in froggy position for the same reason, it's just that a structured waist is not intended for use in froggy position so extra care should be taken.