I read somewhere recently, dunno if it was on here or not that expectorants/cold medicine that thin mucus are bad because, there is a mucosal lining in the intestines and elsewhere in the body, not just in the lungs, and when you take these meds, it thins your intestinal mucous too which is not good obviously..

Is this true? And if so, would herbs like slippery elm etc have the same effect?

I don't know about expectorants but slippery elm is helpful, soothing for leaky gut. One supplement called Perm-A-Vite used to help cure leaky gut has the following ingredients:

Cellulose, Glutamine, Acetyl-D- Glucosamine (shrimp, crab, lobster), Slippery Elm (Ulmus rubra) (Bark) Powder, Stevia (Stevia rebaudiana) (Leaves) Extract, Glandular Complex with Epithelial Growth Factor, MSM (Methylsulfonylmethane)