Re: Why did the Changelings look like Odo?
Alternatively, the Odo-type appearance of the Founders is their default appearance (perhaps a holdover from when they were Solids, too), and Odo himself just hadn't the experience to hold a more complex form than his default. Hence the Founders look like Founders when they're not mimicking another form, and naturally the young Founder Odo looks like one, too.
Also, Salome Jens' earlier appearance as the ancient humanoid in TNG's "The Chase" sort of looks like a precursor to the Odo makeup, so there might be a workable explanation there.
"Every time you think, you weaken the nation." --Moe Howard