Problems with this and that

have an older Sadie (fixed) of 8 years , have a new puppy (Mezo, fixed as of Feb 3rd) he is almost 4 months old.

My problem is that the older dog Sadie, seems to love Mezo, and looks for him when he is gone to the vets. (45 min drive to the vets)
but because Mezo is a puppy she fusses at him. then will snap and even bite.. (no blood)
I am sure I am seeing "who's the boss) Puppy (Mezo) wants to play all day, tries to bite her, has that humping (on her head). bring her toys...etc...I see this as Mezo wants to to the top dog.. So i think I have a power play going on.
We have had to go to water bottles to get Mezo to try to stop his bad habits. (which works for the most part, unless he is in his really excited playing mode)
When Sadie has had enough if this, she will sit next to me or under this desk. and that is my key to keep him away.. sometimes Sadie will play.. but most of the time..99.5 she doesn't want to play or be around him.

My trouble is this.... when it is time to eat.... we have a bowl for Mezo and one for Sadie... they eat in the same room.. Sadie is eating her food and his food.. Mezo is a puppy and will not eat everything all at once, so he eats a few bites and runs plays, comes back and eats again.. until we feed him again.. I am now seeing Sadie eating his and hers.. and she is putting a lot of weight! (Sadie is not a very active dog, hates going outside, she was the runt and is sickly quite often)
If I put one in one room, one in the other, they don't eat, but will look at each other from under the door.. not eating..until they are together again. I am at a loss.
Ps. after the breeder told us about food, we changed foods when Mezo came to live with us, and now feel them both Chicken Soup and they get NuVet plus Supplements.