You have to look at this from your dog's point of view.
In the wild, strange canids are NOT welcomed into other family/pack groups often.
Usually they are accosted and either driven away, or attacked and killed.
Our domestic dogs have had some of this territorial instinct bred out of them, but still, when a new dog comes into your household, most dogs view it as a usurper and a direct threat to their pack status.
It upsets everything, and the newcomer has to be integrated and find his or her place in the pack.
With a puppy, that is going to be on the bottom, esp if the other residents are older dogs.
Too many make the mistake of interfering in the social dance that involves determining rank, and in the older pack members disciplining a brat puppy.
It is common for the higher ranking pack members to stare, be possessive over food and toys, and to show the newbie bodily where she fits in.
You may see ranking pack members turn the puppy upside down, and stand over her and growl, You may even see behavior that really looks and sounds like the older dog is killing the puppy. Usually during these loud corrections, the puppy will be on her back, with the head turned toward the side, presenting her vulnerable areas to her superior.
This is NORMAL dog behavior. I do not interfere unless there is blood. Ranking pack members will routinely discipline puppies in this way. If you interfere, you may get an even stronger response the next time, and you can actually push things into an injury fight when it might not have happened by not letting the behavior and incident run it's natural course.
Being off food is, IMO, likely not related to the arrival of the puppy, and I would have this dog vet checked.
Remember, if you tempt dogs with treats and special food if/when they decide not to eat, you can create an eating disorder that can be very hard to fix. If a dog does not wish to eat, there is a reason.
If there is not medical issue, finicky eaters get LESS food LESS often, not more.
Smart dogs learn to eat when food is presented, because if it's not cleaned up fast, it does not hang around in this house.
