Tosh Ji,
SINGH is NOT "excluisve" to the Khalsa. Lots of Rajputs etc have been using SINGH long before 1699.
Even some SIKHS before the advent of the Khalsa in 1699 are named Singh.
This was CODIFIED - Mandated Name for the Khalsa after 1699 - that is Guru Gobind Singh ji ordered that ALL Males will be SINGH and ALL Females will be KAUR after they have undergone the Khalsa Initiation Ceremony of Khandeh batte dee PAHUL.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=23971
2. It was also mandated that the PAHUL Ceremony is a NEW BEGINNING..a SPIRITUAL REBIRTH..ALL previous castes/gots/creeds/beleifs/etc etc are DISCARDED. The New Spiritual parents are Guru Gobind Singh Ji and Mata sahib kaur Ji....and the New Spiritual Birth Place is KESHGARH, Anandpur Sahib. This means a complete break and a completley New Beginning !!!
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=23971
Thus while NOT all "singh" are Khalsa....ALL KHALSA MUST BE SINGH.