Perhaps not. The problem still arises: that still included Cardassian civilians
On that note...it's quite possible that 31 didn't necessarily plan for the Link itself to be infected.
By that I mean: they infected Odo, on the grounds that he would infect the changelings he'd encounter.
Which changelings would he most likely enounter? The ones on this
side of the wormhole.
Basically...what I'm getting at is that 31 may well have developed the virus as a weapon against the changeling infiltrators
--which Odo would almost certainly help take down.
Those changelings might link with one another--but for obvious reasons, they wouldn't "sneak" out to the Gamma Quadrant to "link up", and then sneak back.
The possibility of it spreading to the Link--perhaps through contacts...or Odo...may therefore have simply been, as far as 31 was concerned, a fortunate happenstance.