Based on his weight, something closer to 15-16 grams would be ideal. However, I don't think 19 grams is TOO big. They certainly can eat bigger prey than they appear to be able to, but that doesn't mean they should. It is fascinating to watch them eat a larger sized prey, quite amazing I think. However, if you can keep his next few meals closer to 15-16 grams that would prob be better.

I would wait a week til next feeding, primarily because he hasn't digested a meal this big yet. Then feed every 7 days.

So far you're having good luck with moving him to a feeding tank, but I would advise just feeding him in his enclosure.

As for switching to rats, you won't have any luck with Petco. I was actually in there yesterday, they only carry SM and LG rats, too big for your guy still (and mine too, who I want to switch to rats as well). There are several good online suppliers (from what I've read, I'm yet to buy online. I buy from Reptile Expos whenever possible) to choose from. Only problem is buying a bulk item your snake hasn't eaten yet. I would try to find a local store that sells feeder rats before buying bulk. It would stink to spend money on 25 or more rats plus shipping and your snake not want to switch.