At wit's end with finding dry cat food! Suggestions welcome...

My cats have been eating Royal Canin dry food, but I have a feeling it's been giving them diarrhea. The vet said there are no parasites/ illness, so it can only be the food. I had them on Hill's z/d for a while, and they didn't really like it, but they ate it mixed with their Royal Canin kibble. This seemed to keep the diarrhea under decent control.

I ran out of the z/d, and since the vet also recommended me to feed Royal Canin Urinary+Calm, I have been feeding them that mixed with their usual kibble. But now 3 of them are having diarrhea! Not to be gross, but it looks like the color of the kibble (the corn this kibble contains in abundance!).

What other dry food have you used successfully to re-establish normal poops in your cats? The kibble size needs to be on the smaller size, as one of the cats has no teeth. Would Hill's be a better choice than Royal Canin? Should I stop giving them the Urinary+Calm? The vet had us on it for the 'calm' part, as my cats don't have urinary issues.