A few questions from a second time mom

AfterAll

Forgive my ignorance... I just can't remember the answers to these questions. 

I found a box of Pampers size 1-2 diapers left over from my son (all sealed).  It says it is up to 15lbs.  I remember only using newborn for less than a month at most because he peed out of them.  My son was 7lbs 13oz at birth and 11.11 by 1 month, so he was a BIG boy.   How early did you start using those Pampers in between sizes like 1-2?  Do I still need 1s?

For the second time moms... if your 1st LO was on anything other than regular formula, like Soy, did you immediately go to Soy or Gentle, or whatever you used with the first kid, with the second?  Or did you try out regular first?

Thanks in advance!