Umm, 19 weeks, I would have hoped you could have picked him up a little quicker than that. Usually 12 weeks is optimum.
I think that extra time with his original Pack, and probably very little exposure to any outside people/other dogs, has made him a little suspicious of those interactions. And the arrival with you, just temporarily interrupted his reaction to that. Shame they didn't tell you what kind of food they were feeding him.
I leave food out 24/7, but, had to go through several kinds to find one that was liked. So, buy small bags for now
On the growling part...
Our Yorkies have a VAST range of personalities. (that's an understatement)
It's good that he may have a protective nature, but you need to be able to control it. A certain amount of correction is warranted, but I think I'd allow a little more time for him to adjust, before I considered this an issue.