Do I need to feed my dogs different types of food? I have 3 dogs. 1 is a small mixed breed who is under 20lb?

I have a larger mixed breed who is 45lbs. I also have a giant breed who is 110lbs. I'm currently feeding proplan shredded dog food. I'm wanting to switch to just proplan. With the shredded my dogs tend to eat only the shredded pieces. The regular comes in large breed adult and small breed adult. Do I need to buy each their own or would my small breed do well on the large breed or vs.? They tend to bowl share and I do not want to separate them because they do well this way and will look for each other if one of them is missing. Before anyone gives me that proplan is crap food and I should reconsider I have tried Innova, Merrick, Canidae, and just about every other premium available where I live. and for some reason proplan is the only food all my dogs do well on. They have good stools, they are healthy, shiny coats, etc. So basically summing up my question is it ok to buy just one type of food for all 3 or should I get a different one for my smaller dog?