Not necessarily. Kirk encountered a number of "shapeshifters" during his TOS journeys, such as the Salt Vampire and Garth of Izar - but he still seemed to consider "true" shapeshifting mythical when he encountered Martia the Chameloid in ST6. The TOS creatures seemed to be "mere" illusionists who made people see what they wanted to see, whereas Martia was truly able to change the shape and size of its ankle and thus escape from a leg iron. The Federation never got to study Martia. Nor did it get a good glimpse at the Vendorian from TAS "The Survivor", perhaps because the species appeared associated with Romulans and might have been native to a region of space out of bounds to Starfleet.
The TNG heroes accumulated more shapeshifter experiences, but curiously, they always came up with a new name for the concept, suggesting diversity and dissimilarity. There were "allasomorphs" and "coalescent organisms", creatures capable of mimicry, but with characteristics very different from those of the eventual Changelings. The allasomorphs came closest to being like Odo, but their "native" form was some sort of a glowing cloud of energy living in an inhospitable environment, not a glowing blob of goo in a shirtsleeves environment.
Probably there are plenty of ways to skin this particular cat, and studying one doesn't necessarily reveal all that much about the other.