I used AIOs on T from newborn, he was 9lb and the AIOs were fine as long as I changed him promptly! I sold all my SIO smalls as they didnt work whatsoever for us til he had really chunked out (I think I started to prefer them after 5 months ish) I have lots of small AIO ittis again this time
We've found the AIO's fit & contain better at a much lower weight than the SIO Eleanor was wearing the AIO's once she hit about 8lbs. We have the odd massive explosion that leaks (but I'm sure no nappy would hold these ones in!!) I'd get AIO's for newborn - then see how the SIO fits as they grow - maybe just getting those in medium for when you need the extra absorbancy coz they are bigger!
I've actually been using medium Itti's on Mira since she was about 8 weeks old and we've had no real containment issues My personal opinion would be don't waste you're money on smalls, just get some mediums