Well you are learning this hobby the same way most of us have! Trial and error and sadly at times a few fish deaths along the way. It is not helped by LFS (Local Fish Stores) selling unsuitable fish to customers just to get a sale) So it is up to us to take our time! I know it is hard to do. Sounds like you are at the LFS regularly, so if you find a fish you like, like your Cory or new Snail, ask lots of questions but doubt what they say and go home and research the species. Make a pact to not buy on impulse or emotion. If you find info that is different, go back and have a conversation with them about it, politely challenge it. This is a great way to learn, and they begin to respect your knowledge and caring for the hobby. Who knows it might lead to a part time job in a fish store some day! You never know where these interactions take us!
When I started the hobby over 40 years ago, the LFS told us to wait 24 hours, put in the fish and hope for the best basically. I can't count the number of times over the years I tore down the tank and scrubbed it clean 2x a year, washed the gravel.... only to experience a new cycle and fish death again.
It's only in the last 2 years being in here at TFK that I finally understand what is going on, and I'm now in my early 50's. Bacteria Cycle + Chlorine = Death. Think of all the fish you will save by learning this stuff now! LOL
I know when you go back to the store, the chosen species might be gone.... but it might be gone anyway (death and your money) and it's easier to deal with disappointment than death/grief.... right?
Pop into chat some evening if you can.... you can ask lots of questions in there!!