I am using Lightroom 6 on PC. The Lr software runs on the PC and the photos are stored on an external hard drive. Can I still use My PC "Pictures" are should I stay away from "Pictures" and dedicate it to Lr?
You can definitely use your Pictures folder.
The Lightroom folder would not take much space as it contains just the Lightroom Catalog and its previews.
You might find Victoria Bampton's FREE LR book to be very helpful:
Her site is packed with lots of useful information: