In the position selection page for moving head and moving mirror fixtures there are selectable shapes. Is there any way to create custom shapes to be embedded in cues on top of the 4 supplied?

no there is no way to make other shapes that the 4 that are given in the software automatically. however you can finely adjust these shapes into various other shapes. You can modify the shapes by using the x scale Y scale speed and phase.
Sincerely,