Hi Jayson.
Are you creating a pure new client or copying an existing one, in your case if there has been customizing performed which has impacted Cross Client tables you will not be able to avoid this. Client specific customizing will not impact your new client unless you are copying an existing one which has been customized.
Can you clarify what you mean by "test the import of developement/customosing transport". Are you testing them from a transport perspective or an application perspective in which can do you have applciation data to test the application logic upon as in functional unit testing.
Thanks
Neil Ryan
SAP Active Global Support