|I haven't used XS Sandys, but it's been discussed in the past that some people like to move from the XS Sandys to the OS. That way you get the trimness of the XS, but some of the benefits of using the OS over the wide weight range once you switch.
Again, I haven't used the XS Sandies, but I would tend to doubt that you'd need the extra absorbancy of a liner/booster. I guess you could buy a few because you'd always be able to lay them into whatever larger diaper you end up going with, but I wouldn't think that you'd need the absorbancy at such a young age. The website lists the absorbancy at 10 oz. Although it's hard to know with breastmilk, the chances that baby is ingesting, and thus peeing, that much are pretty slim, assuming you are changing every few hours or every feed. I used both the S Sandys (12 oz) and the OS (13 oz) on my boys, and neither needed a liner in those (DS#1 did need liners in the OS for a while, but that was significantly later) I did put one in at night once I stopped changing night diapers, but I'm not sure the chances of that are good before 12 lbs.
As you've CDed before, you probably know this, but whatever diaper you go with has to be washed several times before it will reach its maximum absorbancy.