Why your 7 month old has chronic "runs" on Wellness Puppy - who knows. There are a lot of different ingredients in it and it would be a challenge to sort out. Since your other dog is on Acana Grasslands, it seems like a reasonable "all life stages" food to try for your 7 month old. The fat level at 17% is exactly the same as Wellness Puppy. The protein is 33% instead of 28% which is a modest jump. Switching the protein to lamb may or may not be helpful - time will tell.
Protein level doesn't cause diarrhea in and of itself. It's much more complicated than that. Type of protein, all the different ingredients, a company's pre-mix vitamin/mineral profile, etc. all contribute to wether a specific formula will work for an individual dog. And wether or not a specific dog food can be bought at a big-box pet store or a smaller independent pet store has very little to do with how well your dog will do on a specific formula. There are good and not so good choices in both venues.
A tip on switching: do it gradually over 7-10 days for best results; starting old to new in ratios of 75/25, 50/50; 25/75
Also, be careful not to overfeed which can result in diarrhea. The feeding amounts on the bag are often too much.