What you're asking for there is to block all custom fields after the addition of one single short-text field. That's really not a good idea, and not supported - the point of custom field types is that you CAN have many of them.
Jira certainly won't support that off-the-shelf, but it may be possible to create a custom field type that only allows one instance of itself.
I wouldn't bother though - if you really need to have one and only one instance of a custom field, then the most flexible, cheap, fast and effective option is "educate your administrators not to add more than one".
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