I can answer your first question quickly - don't create profiles for people whose names you don't know. In their children's profiles, just leave the name boxes for the mother and father blank. It's best not to write "Unknown" because this gives the person the surname "Unknown" which is hard to undo later if someone comes along who knows what their name was. It happens a lot.
Identifying and tracking unknown great great grandparents is what genealogy is all about. It involves research, much of which you can probably do on line. Common areas of research include Birth, death and marriage data, old newspaper archives, lists and information published by other genealogists and historians on the net, ships of arrival for immigrants, census records, school attendance records, to name a few, not forgetting good old Google!. I would warn against copying family tree information directly from other sites like ancestry.com and familysearch.com. These don't have the rigorous sources requirement that WikiTree does and you could find yourself copying someone else's mistakes. Instead, make a note of the information and treat it as possible clues, which you then verify (or not) by finding sources.
Hope this helps.