I've never foster failed, but have owned and fostered many. Never had an issue. But I did take precautions like muzzling and/or crating when needed,
Feeding separately until I knew the Foster's personality. And the fosters generally had to be taught manners, maybe housebreaking, stairs, glass doors.
Congrats on fostering. Are you in the UK or Australia? Curious since your group is overflowing with greys. Not something you hear nowadays in the US.