Graco Extend2fit car seat for 6 month baby?

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Is Graco extend2fit car seat okay to use for 6 month babies. My baby is 15.5 pounds and 109cm in length. We are deliberating if we should buy a long term stroller to carry both kids, 6 month and 4.5 year or should we go with a short term car seat+ stroller for baby only.