I found Jeri Ryan sexy in her numerous catsuit incarnations, but I didn't find it appropriate for her new station on board. She's a technical member of the Voyager crew.
However, she isn't military academy trained, just like Neelix.
So you can't wear a Starfleet uniform without the proper credentials or approval under stricked curcumstances by the Starfleet military themselves. Seven is a member of the crew like Neelix but still just citizens.