Let me just preface this by saying I don't know anything about formula. But I would stick with the baby formula until he is at least 12 months old. He really does need the extra fat for brain development.
I ff and bf < cronic low supply w/ all 4 boys> I never used the next step or grow fourmulas until they were over 1yr. I would periodicly give them some gro /next step formulas if they went through a non eating phase. ( usualy when they cut molars)
I would stick with your regular similac Its worked for my other 3 and DS4 is currently nursing 2 or 3 x a day but not getting much. He has about 24 oz of formula ( similac sensitve) and we will stick with that till october
I didn't use Go and Grow until DS was close to 1. IIRC, it's geared towards babies 9-24 months. I always thought of it more as toddler formula which is why I waited until 1. He used it until around 18 months.
Are the calories per ounce very different? When my son was on formula he had low weight problems, and my pediatrician was big on keeping him on infant formula well past a year in our case ... I just had the impression that it was more a calorie difference than anything.
I'm a formula feeder (required) and it's probably fine to switch. I'd run it by your pediatrician if you have any questions. I've mostly used Enfamil but Similac is basically the same thing. And toddler formulas aren't a whole lot different than infant ones.