I skimmed through the replies and your update.
Leaking poop at this age is really normal. They do grow out of it in a few weeks.
I wouldn't put much stock into the allergy thing until your baby's poops start becoming more regular and/or there are other signs (increased fussiness, etc). Poop is very hard on skin, and at that age their skin is so incredibly delicate.
For that and when my boys developed a particularly icky rash (like with the stomach flu), I'd put on the original desitin, and then top it with petroleum jelly. The petroleum jelly made a great buffer between the super sticky desitin and the scratchy diaper. It would also prevent the poop from getting stuck to his bum. I can remember trying to scrub off the desitin coated poop from a sore bottom, and it isn't fun! It also meant that the desitin stayed on the important parts rather than rubbed off.
Wife to dh, Mommy to my heavenly angel, J (06), and my earthly angels, S (07) and E (10)