@ bepster, if I recall correctly, still some rights remain to the individual creator who did the design, despite the fact that she/he was an employee in a design studio. To be on the safe side though, I asked my lawyer and I am waiting for his response.
@sketchgrad, thank you as well for your time to share your experience. Perhaps I should be more descriptive with text like the one you mention. I was planning to write something like “In collaboration with XXXXXX design” but this may give the falls impression that my design studio collaborated with my ex-design studio as equal partners.