Yes, you can rename the xaml file by either closing it renaming it manually in the folder where it located OR in Studio you can go to the “Project” pane (bottom-left), and right click on the workflow, then choose “Rename”. The next thing you would want to do is change the Main xaml by either doing so manually in the project.json file (using Notepad) OR in Studio you can go to the “Project” pane (bottom-left), and right click on the workflow, then choose “Set as Main”. Lastly, you will probably want to also rename the parent scope as desired by clicking in the white space outside the most exterior sequence box to select the most exterior scope, then in the “Properties” pane (bottom-right) and change “DisplayName”.
That is correct if you want to start a new project. If you want to add a workflow to an existing project then, from the “New” dropdown in the menu ribbon, choose either Sequence or Flowchart
I hope this answers your questions.