The Find/Replace Tab of the Find and Replace tool window is supposed to be a superset of the mini-find dialog. Both these dialogs can search <Workspace> and <All Buffers>. It would very odd for the bigger dialog to have less features.
The Find Tab defaults to <Current Buffer> if I start it with a clean config, open a workspace, and run the "tool-gui-find" command (or configure gui-find to display it).