Here is one way you can do a circular picture in picture:

I attached a black and white image of a feathered circle to this post - import this into your production. Next, open the settings of the input you want to put in the circle. If you will still need to use the source without the circle, first make a virtual input of it and open its settings instead. On the left side menu, select "Colour Key". In the settings, there is a box labeled "Luma Key" with a dropdown for the Key/Fill Input. Select the circle from the dropdown, and the result will be a feathered circle of the input on a transparent background. From there, you can use the input either on a key or as part of a Multiview and it will retain the transparency.

You can take the same approach to create different picture in picture shapes; just create a black and white image of the shape you want (with white being video and black being transparent). For no feathering, create the source image without any gradients between the colors.
Feathered Circle 1080.png (98kb) downloaded 30 time(s).