Re: changing the look of forums: need to add code into tikineat.css from another template.
Neither of these themes is planned to be in the next major update of TikiWiki so, no, there won't be any conflicts. Themes are generally transitioning to more modern layout methods, avoiding using tables for page layout, avoiding specifying font sizes in pixels, and so on. As upgrade time approaches, there will be more information on forward migration options for sites using older themes.
-- Gary