I believe the conventional thinking on this matter is that Changelings could shapeshift fast enough to avoid assimilation. However, if a quantum stasis field were used to hold a Changeling in one form long enough to assimilate them, it might work. But would the QSF have to be held on them indefinitely, or just temporarily? Indefinitely would kill the Changeling, defeating the point of assimilating them.
And Jem'Hadar drones would eliminate the need for maturation chambers. A Jem'Hadar grows to adult size in three days. They already have built in defenses that drones lack, like the shroud, and are more durable than most humanoids.