I mix Canidae w/ any Merricks flavor. Right now I am using puppy plate, but I have also bought Granny's pot pie.
If you want to feed Canidae just mix it with another that they will eat.

This way they are getting 2 high quality foods.
I think all dogs have different stools on different foods. It just depends on how their bodies digest it. Just change the food slowly so they don't get to runny (they may at first just because of the change). If they stay that way then switch food for sure. Good luck!