CSR was wrong, wrong, wrong!!
you will be fine. i used a tmo unlocked pearl on att from the VERY beginning (last august a year ago) and my husband is still using his that i bought (tmo bb, branded and all -- unlocked on the ATT network) and we have NEVER had issues.
call back, don't tell them its an unlocked bb...add bb plan and you will be good to go
and welcome to pinstack