yep, changing their food al the time will cause runny poos. I heard science diet is ok, and it's affordable. I use Authority from petsmart. And the girls like it.
Try to stick with one type of brand and keep an eye on their coat and poop. If their fur is greasy it's not the cat but the food. And if they really stink when they poop, again it's the food. Not the cat...
Some cats do well on cheap food, It all depends on the kitty.