I thought the infant insert was just that? Something to make them fit better while they are teeny tiny? I would think if you removed the insert and she doesn't seem way too small for it, it would be ok? I don't have a bub but a friend with the same seat as you removed the insert because she has a chubby bubba who is also very tall. She reached the markers at 4ish months. She is still RF at nearly 8 months now but will probaby only have another month or two before she hits the markers again