To be honest, I think you are worrying too much.
Each of us takes a risk when we get new livestock and we can only do so much, like qt'ing the new guys. I would buy only healthy looking fish from a store that is clean with no dead fish in the tanks and no outward signs of disease. Spend a good amount of time watching the fish you have in mind, I spend like a half hour or more checking out a new tank candidate.
Do the best you can do, some things are out of our control and unfortunately makes us go into reactive mode rather than proactive.
Hope that all didn't sound mean! LOL I just want you to enjoy your tanks and not worry so much.