Ok, I didn't see this on any other posting. Just putting this out there: The Founders are actually the elder race from the TNG episode "The Chase", but evolved to transcend their solid origins.
That's possible. The Female Changeling once told Odo that the Founders had, at one time in the far distant past, been solids but evolved into their current form.
Nothing ever stated that the Founders were humanoids in the past. Odo only looked humanoid because he was found and raised by them. The female Founder (the Foundress? lol) chose to look humanoid when interacting with Odo and the humanoid solids.
The lines mentioned were as follows: So yes, they were 'mono-forms', but one could argue that she never said they were humanoid mono-forms.