Power CMS 2.1.1 is a solution pack, an already created PHP Application, it is not an Extension that would cause conflict with Validation Toolkit.
You may have other extensions that will conflict with validation toolkit, for example Form Toolkit or Css form Builder, in this case you will not be able to have both Validation toolkit and the newer extensions installed.
If you need to be able to use validation toolkit to edit existing pages, you should create a new user on your system, and install validation toolkit under that new user, you will be able to login to that new user account to use validation toolkit to edit your existing forms. the new user will allow you to have one user where the newer extensions are installed, and another user where the older extensions are installed.