Some months ago, I had this problem:

I need a custom parent type field (textbox, select, clear) linked only with some modules (i.e. accounts, contacts, leads and prospects) but want to mantain intact parent_type list in order to use it in other modules.

Jyim nicely reply me a message with the answer. I transcript here her instructions.

What you need to do is create a custom drop down list containing only the modules that you want in the drop down. For example, you can create a custom drop down list named parent_type_display2, and include only Accounts and Leads in the list. Be sure to use the same "Item Name" and "Display Label" that appear for these modules in the parent_type_display drop down list.

Then, after you create the Flex Relate field for a module, go to the fields_meta_data table and find the row that contains [name_of_module]parent_name. For example, if I created the custom field in the Cases module, I'd find the row titled "Casesparent_name".

Then, edit the ext1 value so that instead of "parent_type_display" it is the name of the custom drop down list that you created. In this example, I'd replace it with "parent_type_display2".

After you add the custom field to the module's edit view layout and Save & Deploy, you will see the custom drop down list associated with it.
It works perfectly. I hope somebaody can use it as I did. If have troubles, post in order to help you, I know this works.

Sugar 5.0a
Win server 2003
apache 2.2.4
php 5