No shoes for babies too small to be pulling up and standing on their own. Not only is it pointless, it's bad for their feet, too. Just get some coordinating socks.
BTW, the average US singleton baby is around 8 lbs. at birth these days, so a 3-6 mo. outfit is usually only a little loose on a newborn. Buying that size is not buying ahead to grow into by much, so plan the seasonality accordingly.