My best advice to you is to take things very very slow. I know it's hard I'm the least patient person on earth. Buy a few fish at a time, qt, treat full term with one medication at a time, then remove meds and observe 30 days before adding.
My arm, nets, magnets, water siphons and hose, splash back in to my mixed fish food, etc. have all caused cross contamination in my tanks in the past year. I too am very busy and mentally idle half the time so I made stupid mistakes.
It happens. Ich management is nearly impossible to utilize these days with stronger strains of parasites and velvet so prevalent. If you're buying a lot of fish you're absoslutely going to get exposed to everything in the book.
Add the fact that you and I like to keep expensive and difficult to keep fish, ich management never worked for these fish by and large.
Good luck, take it slow and you'll be OK