Let me mention first that there is an alternative to the generic Create New button for creating a related object. This involves adding an operation to the object form (that can be displayed above or below the form) to start a process that creates a related object. The advantage is that you can specify a more meaningful name for the operation compared to the generic Create New button.
For example, in the Library sample application the form for object Member displays operation 'New contact note', which starts process CreateMemberContactNote containing a single rule:
ENTER NEW ContactNote WITH ContactNote.Correspondent=Member
which displays a form for the new contact note to the user.
You can use this technique to create related objects with or without displaying the new object form to the users. You need to define two more groups in addition to the one you have now. One of the new groups (say Group2) will contain a subset of objects in the main group that require their form to be displayed. You then add a process and an operation as described above for the user to create one of these objects. Since the ENTER NEW action will refer to the group name, the system will show the object selection dialog where the user will select the desired object type (just as it happens now when the user clicks the Create New button).
The other new group (say Group3) will contain a subset of objects in the main group that do not require a form to be displayed. You then add another process and another operation as described above, except that you need to use action CREATE instead of ENTER NEW, which creates an object without displaying a form to the user.
In this solution, however, the 'CREATE Group3' bit will not work at the moment because the CREATE action currently does not accept a group name - only an object name. We will remove this limitation in version 1.6, which is due for release within the next few weeks.
You can go ahead and test the described solution using ENTER NEW in both cases for now. When version 1.6 is released you would simply replace ENTER NEW with CREATE in the second case.