I'm not a super big Stress Coat fan. I "think" it may "gunk up" gills & slime coat, etc. I use Prime as my dechlor, although I've never had an issue with fish from an lfs that squirts some Stress Coat into the bag.
I think you need to relax, losing a fish is stressful. Just observe you fish for now, water changes are always good. Do that if you need to "do something" before dumping a bunch of stuff in your tank.
Sometimes we just buy an unhealthy fish, don't panic! The rest may be fine!
I've never treated a tank with inverts I actually wanted to live (shrimp, ramshorn, mystery & nerites snails). Just research a lot before doing anything rash.
While 6 shoaling or schooling fish is a "minimum", there's no need to freak out and add another. Get the tank & fish health stable before anything else! I'd go for at least 2-4 months of tank stability before adding any more animals. You want a long term healthy tank.
Keep us up to date on how your tank is doing in the short & longer term, or if other problems show up. Good luck!