Congratulations on your new addition
She is a cutie and definitely sounds like she is in better hands now. At 2 years old, I wouldn't be too concerned about her getting any larger. For my two, I measure their food with a shot glass. They each get one scoop in the morning and one in the evening. My two weigh 7 and 5 lbs. I wouldn't let them share because you won't be able to keep track of how much they are actually eating.
There are a lot of opinions on food here, but I personally think grain free kibble is great. Some people home cook, feed raw, use dehydrated... But we travel too much and that wouldn't work for us. Do some research and figure out what is best for your pups
I definitely think you can do better than Purina One though!