Feeding a new kibble with the old kibble

[mscat]

I started my two girls on eating Evo , I have been mixing the two together. Wellness small breed. What I have noticed is that they are pooping less , ever since I but them on Evo small bites , red meat. I realise Evo has a very high protein count. I made the decision to do this because my shih tzu could use to put on a pund or 2. Suzi our yorkie is fine weight wise, but i do not want her to get any heavier. I free feed both of them.
I thought of buying some canned food wellness, the one that has 95% protein, and just feed the shih Tzu that a couple times a day. But Hopefully the Evo will help her more then having to feed straight from a high quality canned food. I would then need to feed them separately. Suzi has her coat in and she looks good, but I think it does make her look heavier with the longer coat. If she were to be shaved down or in a puppy cut, I think she would look a lot smaller.
I also noticed that her coat coloring is still changing at 3 years old. She is getting a lot more sliver color on her back, and on the top of her head it is a soot color. And very light .