Personally I think rather than adding things to the diet to try and get rid of the slushy poo you should look at what you are feeding and quantities. Why are you feeding both a dry and a wet food? Either is perfectly adequate, but juggling the two can make it difficult to get the balance right. I'd try feeding just the dry but keep to the recommended feeding rate for the dog's weight - maybe even slightly under. I've got a feeling you are possibly overfeeding, and that would certainly cause the problems you write about.