A.C.
As far as the infant car seat goes, the rule is that their head has to be an inch down from the top of the back (where their head rests). It does not matter if his feet or legs hang over, that has nothing to do with the safety of it. If the "bigger" car seat says it's a rear facing and up to 35 lbs rear facing, then you can put him in it. I would definitely read the manu. directions for both car seats just to be on the safe side. If you've lost them, you can usually find them online or email/call the company and they will send you a new copy. Hope this helps!