We got our Ergo, with insert, just at the edge of the recommended range for the insert -- I think DD was 13 weeks. We did use the insert a few times, but by 15 weeks we were just doing a straight front carry.
The insert is pretty confusing; it's hard to imagine using it for a baby who doesn't _need_ it. I'd probably recommend not bothering -- you wouldn't use it for long, and there are workarounds. When it's winter (well, okay, soon it won't be! perhaps where you are it already is over!) a puffy snowsuit can play the same role in letting you carry the baby in a "sling" position. Or, as long as your baby has good head control (DD did quite early on) you can let him/her keep one or both feet tucked in at the bottom, instead of splayed out on either side of you... that was definitely more comfortable for DD at first.