You can use it on any seedling. How effective it may be is a whole other discussion.

Damp off fungus quickly circulates through the entire plant and destroys the cellular structure of the stem. So the odds of saving the plant is minimal and if it does survive it is with a very weakened circulatory system and chronic health issues.

Damp-off is easily prevented in the first place with proper watering techniques and adequate air circulation so that is the best route to take.

As to the ants - assuming they are not actually fungus gnats, I assume these are plants in the ground rather than in containers? Containers aren't normally bothered by ants. The solution is to discover why and where the ants are attracted from and eliminate that causes/source. Most ants are considered garden neutrals - do little to any harm to plants - so more details would be needed to address that issue.

Dave