We know the Founders impersonated key figures in their espionage efforts. We know they replaced several people from various governments.
With In Purgatory's Shadow we learn who some of those people were. What I don't understand is why the Dominion kept them alive. The double was in place, they didn't believe in fairplay, and they really had no need to keep these Solids around anymore. Of course it's necessary for the writers to keep these guys alive, but in-universe, the Founders don't give a flying fish about them. Though I guess they might want to do some sort of mind probing experiments for intelligence as we'd seen before.
A Ferengi Changeling might have been interesting-- and nobody discovers him until "Rom" beats up Quark.
I liked the impersonated Commander Lovok of the T'alShiar. Too bad he blew his cover.
Bashir could've been spared because he was with Odo in "Broken Link" and perhaps they couldn't bring themselves to killing someone a fellow Changeling, even if a pariah, can trust or confide in. Maybe they wanted to know what made him ("that solid") so special/different. Maybe some end up used as slaves or some are kept alive to psychologically break them, like a form of high profile prisoners (i.e. they kill people they deem of low value but hold captive people of high value). Tain had the gall to lead an extermination force against them.
Yeah, a Ferengi Changeling would've been interesting. Thinking about the various crews who ended up possessed or replaced by impostors at one time or another, a Quark impostor or Ferengi Changeling would've been interesting, though I think the Ferengi weren't seen as a threat. They would easily roll over to the Dominion since all they care about is money and commerce and the Dominion lets that go on, so long as they have oversight.