I feed all of my dogs, no matter the age,
Eagle Pack Holistic Chicken & Rice Adult & Puppy Formula. I have never once had a problem with "non-firm" poo here. Might want to either give them a call for a sample or heck, let me know and I'll send you some to try if you'd like.
Eagle Pack doesn't have a smell, isn't oily, can't find a thing wrong with it really. All the dogs just love the stuff. It's great because more of it is digested and there's actually LESS poo with this stuff than with most of the dog food out there these days.
Here's
Eagle Pack's website.
They also have a product that is designed to help with food transitions. Here's a link to that.
Holistic Transitions
Or heck...you may just need this one....
Holistic Solution - designed to eliminate loose stool & upset stomach.