Can a lack of insoluble fiber in diet for almost 8 months cause thin stools? and is a mushy but still formed stool considered loose or normal stool?

No: lack of fiber will make the stool harder and thicker fiber is important for a healthy vowels the stool you have is a reflection of your diet eat more healthy hydrate well and exercise

See a doctor: Change in caliber of the stools such as thinning can be a warning sign for colon cancer. I would not attribute this to lack of soluble fiber. Please see a doctor for examination and appropriate tests, such as colonoscopy.