In speech, citation forms are usually heard when they are pitch accented or emphasized. In which case, yes, apply the standard formula (sibilant +'s = sibilant + i + s/z). Besides this, there is another phenomenon, which kinda elides the similar looking syllable.
career center >cur center (check
Language Log » Why it can be hard to wreck a nice beach )
social security > so' security (as heard in the financial media)
absolutely right > absly right
Bales's family > Bales family
One can ask "how to distinguish social security from campus security, career center from computer center, Bale's family from Bales's family". You don't need to emphasize these words, if the audience can figure out--this is an easy way of describing. There are books that deal with emphasis, focus (both broad and narrow), and accent. Just because some accent reduction course or ESL book does not describe these phenomena, it doesn't mean that such phenomena do not exist.