. . .odo explicity mentions that internal sensors can't detect his people unless they are in their founder birthday suits, he states if a founder is a rock you'll detect a rock, if a founder is a humanoid you'll detect a humanoid. With that said we see the founders actually become what they shap shift into. If the borg assimilated a founder in humanoid form then it would be like assimilating any other humanoid. At that point you would have an assimilated founder. . .
I think there's a flaw in this argument. A changeling has the ability to somehow cause false sensor readings. He might look like a duck, walk like a duck, and quack like a duck, but that doesn't make him a duck. He's a changeling pretending to be a duck, camouflaging his true form.
If a Borg drone tried to assimilate a disguised changeling, at the moment of physical contact, the changeling could almost instantly shift into something else, including natural liquid form. There's no evidence that a disguised founder actually has the internal organs or molecular structure of an impersonated being/object. But they can fool sensors well enough to show that they do.