You really have to decide whether you want to go with quarantine approach or not. It sounds like your LFS gave you really bad advice. It is not difficult to set up a quarantine tank. All you need is a cheap tank from PetCo or you can get a used one online, HOB filter such as the Tidal line from Seachem (I use a Tidal 55 on a 10 gallon--hope to upgrade to a long format 20 gallon QT tank), and cheap heater. Add in a reliable autofeeder, some PVC pipe for hiding areas, and Hanna copper checker and you are good to go.
I know it seems painful but I have a similar sized tank to you and have had to pull out all the rockwork twice in the last six months to catch fish or do other painful maintenance. If you don't want to set up a separate quarantine tank you could theoretically pull out all the rockwork and sand and run copper and other meds in your main tank, then add back the aquascape once the fish have been treated.
I know it seems painful but I have a similar sized tank to you and have had to pull out all the rockwork twice in the last six months to catch fish or do other painful maintenance. If you don't want to set up a separate quarantine tank you could theoretically pull out all the rockwork and sand and run copper and other meds in your main tank, then add back the aquascape once the fish have been treated.