Fertilome Weed Out With Crabgrass Killer RTU will not kill fescue as it is labeled to be used on both warm and cool season lawns for crabgrass and broadleaf weed control.
Since tall fescue is a cool season grass type, the best way to eradicate it in a cool season lawn and not damage your lawn is going to be to spot treat using a non selective such as Roundup QuickPro and reseed after 2 weeks.
We do not recommend mixing Fertilome Weed Out With Crabgrass Killer RTU with an insecticide. This product is a ready to use and is used as is. Also typically not recommended to tank mix the 2 types of products as each contain different properties that could degrade the effectiveness of the active ingredients