> I need TikiWiki to recognise the copyright owner (creator) of Wikipages so that only him/her can edit it, but I do not want to give them total admin rights to the pages they create.
There is an option under "Admin > Wiki" (right column of options) that you can toggle off to avoid this.
> A second question is about levels. I see TikiWiki has 4 default levels (Anonymous, Registered, Editors and Admins). I know how to create new levels, but can I delete, modify or give them a different set of permissions than the default ones? If yes, where is this done?
No idea myself. Did you try "trial and error" through the drop down boxes under permission settings for groups? ;-) (that's what I would try myself)
Hope other person answer beter to your 2nd question.