Clearly, the Jem'Hadar or the Tosk are not shapeshifters: they don't blend into the background by physically becoming the background, or they would be far more irresistible a force, far less often daunted by obstacles designed to stop mere solid humanoids. But we have seen shapeshifting or camouflaging of another sort, that created by telepathic illusion. The Salt Vampire worked on that principle: never "actually" looking any different from its natural, hairy form, but instead "pretending" to be many sorts of other things, often simultaneously being different things to different observers.
The Jem'Hadar are likely to be psychic as well, as their cloaks can efficiently hide random pieces of equipment, such as guns or clothing. They just instill in our heads the idea that there's nothing to be seen there. A tricorder might spot them easily enough - we never really see that not
happen! Or then they might instill in our heads the idea that the tricorder is showing nothing...
It's just that the Jem'Hadar and the Tosk are pretty primitive in what they do, only ever managing to pretend "not-being-hereness". The Salt Vampire is more refined, capable of pretending to be whatever the observer wants to see. We never learn whether there is actual intelligence behind that pretense, though: the one Salt Vampire we witness acts like an animal driven by instinct, and seals its fate by the failure to demonstrate much in the way of intelligence or strategy.
I guess the interesting question is, how much of the Founder ability to shapeshift/become invisible is psychic, and how much of it is physical. We saw Martia in ST6 actually changing her size so that a shackle that originally gripped her leg tightly suddenly became several sizes too big to hold her. We haven't actually seen the Founders do that exact sort of trick, but Odo has squeezed through some tight holes; has exerted superhuman forces on objects; and has become airborne - all of these are tricks a humanoid or an amber mass of goo would be hard pressed to achieve with mere telepathic tomfoolery.
Plus, it would make sense for the Founders not to give their own
sort of public-image-manipulating ability to their cloned warriors. Probably the Founders actually change shape, then, while the Jem'Hadar and the Tosk just make us forget they are there.