I have been "defecated on" while handling my snakes, and I think that it was just bad timing. In all but one case. This case being, the WC adult male BP I rescued. He didn't really defecate on me, but he let out this "musky" smelling stuff. That's a defensive emission, not a "normal" emission. I never knew BPs could do that until it happened. He is, after all, an animal caught in the wild as an adult. But if your snake merely passed normal feces while you were handling, then it was probably just a coincidence.
It is good to hear that your new guy is eating. Was it a fuzzy mouse, or a pinkie? I would not feed pinkies; fuzzies are more nutritious because of their more-formed bones.
I highly recommend asking the store to get in larger mice, because one larger meal is better than several small sporadic feedings. I usually don't see a distinct bulge in snakes after feeding unless the animal is underweight. Or you could order some frozen mice from a website like mieconice.com. And congrats on havint a f/t eater! Many people go through alot of strife trying to get their BP switched frrom live to thawed! Also, you are right in your feeding timing; it is better to feed BPs after dark, since they are nocturnal.

I use "astruturf" substrate, and I like it. Much more attratcive than newspaper, although you do have to put in the extra effort to wash it when it is soiled unless you plan on spending big bucks on new astroturf every few weeks! I keep two pieces per enclosure, so that I can replace the soiled one while it's cleaned. The kind I use has both brown and beige colors, and I bought it at a hardware store for something like 30 cents a square foot. They sell green too. Good luck with your BP! I never knew that petco sells captive bred BPs. Maybe captive hatched?