Hi Bob,
A present, you cannot change the default Task Type value, however this is changing in a couple of different ways with our upcoming Winter release.
First, the system default Task Type will change to Fixed Duration, which is the more common way Daptiv users schedule tasks. Second, the Create Task option will "remember" what each user selected from the previous Task creation, effectively becoming the default setting for each specific user.
Regarding the modification of standard applications, you can add custom fields to any of the applications, but you cannot alter the default fields or functionality. You will see greater flexibility around the applications in this regard in future releases, but for the time being you have the option to add custom fields.
Finally, unless you are using the Quick Create, which defaults to the current date, the Planned Start date for the Task will default to the Project Planned Start Date, which is set at project creation and can be modified by editing the project profile.