I have added rich text component on partial design and assigned that partial design to page design and further added that page design to page. However, when I tried to edit the rich text field from the page it's not editable.
Please suggest what should be the default behaviour? Is the field editable on the page or should it be changed only from the partial design itself?
This is the default behaviour. Components that get placed on a partial design are not editable on pages that use these. I assume this is done because you would be editing all the pages that use the page/partial design - probably without knowing it.
So you should reconsider your content & design architecture. Does the content need to be in a rich text component or can it come from the page? You can edit content from a "page content" component on the pages (even if the component was set in a partial design) but those limit your options for personalization.
In version 10.1 Page Branches are an option too. They work like templates that can be used to create pages with components and all - so your author has a starting point but can change everything. But if you change the template, you do not change all the pages.
So it's now back up to you to decide what is important and how this best fits your solution.
