I am breastfeeding my ds (6 weeks) he has occasional formula top-ups and bottles when I am out (max 2 bottles per week usually) I have given him Sma first stage ever since the night he was born and he doesn't have any ill effects (although has never had very much) I always use the ready made cartons. however my friend has given me a couple of ready made bottles of cow&gate her baby doesn't use and I was wondering if it's ok to switch between brands or formula for no good reason other than its free/cheaper? It's only for occasional top ups so it's not like he's used to one brand. Plus I reckon they're all very similar but packaged differently. Any experiences??