Validation rules in Salesforce are able to reference the current user, along with their profile and role. This means you can setup the validation rule so it will only be enforced for certain users.
Also: if you don't absolutely need to have "Choose One" as the default, you can leave the default value blank, and only set the field to required on the page layout for those users.
Please note: In both situations, the first user to edit the record (after Marketo syncs it) will be forced to select a value in that field before they can save (unless that user/profile/role is excluded, of course)