adding columns to the data base after your pages are created will not cause any issues, only removing columns will cause issues.
However, when you add a column to the database don't expect any existing pages to automatically recognize the new column.
in the case of an insert page, you would need to add a new form element and edit the insert behavior if you want the insert to manage the newly added columns.
The question you ask does not really have 1 answer to it. It is always best to have the database structure completed before you start development.
when you get to the point of adding features to a web site, you should consider whether updating the existing site is the best approach or is time to Upgrade the site to V2 by completely rebuilding it.
it may be better to rebuild the site into a new version rather than add to an existing house of cards.