Is submuscosal fibroid and subserosal fibroid harmful?

Not necessarily. Many, many women have fibroids as they get older. Most don't even know they have them. If you are having symptoms due to them, like heavy periods, etc., then they may be worth treating. Discuss your options with your doctor. Very very rarely, a fibroid can become cancerous. See http://womenshealth.gov/publications/our-publications/fact-sheet/uterine-fibroids.html#g.
No. Fibroids may or may not cause symptoms. Submucosal fibroids found along lining of uterus can cause heavy menstruals which can lead to anemia (low iron). Subserosal fibroids found on outer part of uterus can cause bulk-related symptoms by pushing on whats next to them (exs. pelvic nerves to cause pain, bladder to cause increased urinary frequency). At 29, fibroids can cause fertility issues as well.