We don't know, as we never saw a Changeling interact with the Dominion minions - besides this single case of the Female Founder interacting with both Odo and the minions on DS9 and then getting stuck with that form. I wouldn't jump to the conclusion that the Female Founder's form was in any way common among the Founders. It probably wasn't used in any occasion that did not involve Odo and his Doctor Mora looks somehow.but the Vorta have seen the Changelings before they ever met Odo, so what did they look like then?
All this is explained as Odo imitating Dr. Mora Pol. He got the looks and the gender from there, but initially fumbled the Bajoran nose (and even commented on the difficulty of doing it right). He then grew up a bit and decided he didn't need to perfectly mimic anybody, so he kept the "unfinished Dr. Mora" look because it was the closest thing he had to a personal look.- was Odo a kind of default humanoid form that changelings adopted and that's why he took that form (when accurately mimicking someone wasn't an option) - and what was it that determined gender -
Quite a few "liquid creatures" (gooey pools of barely differentiated cells) here on Earth have genders, the usual way: one carries the XX chromosome pair, the other has the XY. Although there are variations on that.how do liquid creatures have genders at all
It was established in "A Man Alone" already that Odo has DNA. We don't know if all Founders have it, or if it's merely something Odo instinctively imitated when deciding to be like Dr. Mora. But if it's a general Founder thing, then it probably follows that the gender thing also works like our X and Y chromosomes setup - it's something of a package. And TNG "The Chase" did establish that (nearly?) all humanoid life in our galaxy is biologically related, the result of seeding by an ancient Solid species that may or may not be related to the Founders. (The fact that a member of this species was played by the same actress who played the Female Founder is hardly conclusive!)
We don't know if the Founders evolved from Solids. Yes, this was claimed once, but the Founders and their minions appear to be lying bastards through and through...- was it a choice or do the founders have genders when they're in liquid form - presumeably the female changeling had breasts because the founders did have solid gender forms (possibly that they evolved form)
Why the Female Founder grew breasts, I have no real idea. Several Founders did when they first saw Odo in "The Search". Then again, they also saw Kira. Perhaps they realized that Odo was comfortable with his humanoid looks and would be even more comfortable if the entire Founder "species" adopted humanoid-like looks - which means they would have breasts and penises, or at least protrusions that would create that illusion beneath the "clothing" that the Founders also created.
Timo Saloniemi