We have been using Mediums from about 3 months - when was the nappy fairy? Without a single poo leak. I would definately give the smalls a miss, really no point.
I have extra inserts but don't often use them, either because he poos so I can't reuse it or because it is too much of a faff to change the inserts and I risk him weeing everywhere while I do it
If I am out for the day, I tend to take a few extra sets as they are smaller than a whole nappy so it is handy then but most of the time if I am at home I just change the whole nappy. Sometimes I have swapped the nappy and made up the previous one ready for the next change iyswim but most of the time I can't be bothered and bung it in the bin.