Actually, the only time we see other Founders imitating Odo's looks is when they are with
Odo. So the "they want to make Odo feel comfortable" theory really holds water rather well.
That is, there is this one exception, but it's easily explained away. Namely, sure we see the Salome Jens Founder in her Odo-like guise even when there are only Dominion minions around. But we only see her in such guise in such environs after she has been infected with the S31 bioweapon
. She probably is already feeling the first symptoms of the disease, and finds it painful to shapeshift, so she decides to hold on to a single shape most of the time. The Odo look is the one the minions are familiar with already, so it will be accepted with the minimum of explanations. Towards the end of the show, the Founder is probably stuck with that look, unable to shapeshift at all, and making all sorts of silly excuses that Weoyun easily sees through and even the Cardassian allies are beginning to suspect.