There's no evidence that all 76 people supposedly involved in the Founders' disease development were all S31. Research into Odo's abilities and nature could've been corrupted into a bio weapon.
If research into white mice's genome or proteome were corrupted into a weapon to eradicate them, that doesn't make destruction the original aim of the research. We know S31 can pull people into its activities despite their wishes, like Bashir was. Just because he was involved in a successful S31 operation doesn't make him an operative, even if Sloan considered him one. How many of those 76 were willing participants, or even knowledgeable about the purpose of the research and S31's goal?