One also has to remember that these represent somewhat different categories of shapeshifter.
Martia was able to actually alter her (its?) shape, becoming smaller than a shackle and so forth. The Salt Vampire did none of that: she (?) only created mental illusions, sometimes of different things to different people in the same room. Garth never demonstrated the ability to alter shape (except from one humanoid to another), but he never demonstrated the ability to be different things to different people simultaneously, either. However, he's IMHO a likelier candidate for another Salt Vampire than for another Martia. The Kelvans in turn were explicitly said to be doing bodily adaptations (which made them physically vulnerable), but apparently in a manner that transcended mere biology. They didn't change in Kirk's presence, either.
Kirk would thus only have encountered one "true" shapeshifter in his career: the Vendorian from TAS "Survivor". That creature was able to fool people into thinking it was a table - but perhaps that would still be possible as a mental trick. However, it (?) was also able to become a deflector shield, or perhaps a component that fixed the sabotaged deflector shield (a more realistic possibility that the episode certainly doesn't contradict), and that couldn't have been a mental trick. Or if it was, then it was a poorly chosen mental trick, because if the Vendorian could trick the Romulans that effectively, it (?) should have tricked them into not inflicting lethal wounds...
Another shapeshifter of sorts from TAS: Ari Ibn Bem, a colony creature whose components could mimic humanoid head, torso and feet. Or at least that's my interpretation of it, although Bem could also have consisted of a head, torso and feet instead. It just wouldn't make much sense, biology-wise or plotwise. In any case, Kirk's time with Bem wasn't a happy one, either... Clearly, shapeshifters are shifty characters!
Timo Saloniemi
