Is it not explicitly stated that mature Shapeshifters--the Founders, not Odo or Laas--prefer to remain in the link, unshaped?
"The Search" suggests Changelings greatly enjoy experimenting being other things, such as rocks, seagulls, fire or for all we know jealousy or lateness. This is never stated to be something only children would engage in. We see Odo the child being guided to do so by Female Founder the adult, yes - but Laas the teenager (?) also shapeshifts a lot.
The Female Founder later blatantly lies that she wants to maintain a specific shape, to cover up the fact that she no longer can shapeshift. But that's not a convincing argument for adults not doing it in general.
The O'Brien look-alike on Earth suggests that there were only a few (though that cannot be entirely trusted).
Indeed, if somebody like that stated there are four of them, I would at first assume that the true number is anything
but four. But then I'd start doubting myself.
Wasn't it explicitly stated in the show that the virus was designed to move more slowly in Odo because he was meant as a carrier?
I can't quite spot such a reference. "Extreme Measures"? Something else?
Timo Saloniemi