Which string identifiers for field types do exist?
I don't know if any enum exists (couldn't find it in source), but you could have a look at the contents of the craft/app/fieldtypes folder (and ignore BaseFieldtypes, DataModels and interfaces) to find the identifiers:


UsersFieldType.php
TagsFieldType.php
TableFieldType.php
RichTextFieldType.php
RadioButtonsFieldType.php
PositionSelectFieldType.php
PlainTextFieldType.php
NumberFieldType.php
MultiSelectFieldType.php
MatrixFieldType.php
LightswitchFieldType.php
EntriesFieldType.php
DropdownFieldType.php
DateFieldType.php
ColorFieldType.php
CheckboxesFieldType.php
CategoriesFieldType.php
AssetsFieldType.php


String identifiers correspond to the file names (without "FieldType.php", obviously).