There are literal comments on here of it being difficult to feed and produces too much waste. Neither are true.I think most of us are absolutely aware of what the fish is and what it does. We are also aware of recommended swimming space advised. Min 30g (read) 36 inches is for the optimal overall health of the animal. You didn’t mention your tank size or your other inhabitants. Another potential issue with this stocking are fish known to display extreme territorial aggression. The filefish, or any open water swimmer, will likely have very limited swimming space caught between Scylla and Charybdis. Swim one direction and a clown pair is chasing it, swim the other way and a gramma has its mouth open wide enough to swallow a conch. The recommendations are not arbitrary and aren’t being made without thought and consideration for both health of livestock and success of the tank. If it were just the filefish and maybe a shrimp goby and a firefish or a clown goby, a captive bred filefish would possibly be ok.
The OP has many fish on the list that are recommended for larger tanks and/or won’t house well together. In all honesty, the colorful tank they initially described is not attainable in a 20.
I would have to look back what I kept it with in a nano but definitely a clownfish.
It’s a macro algae tank, it won’t be sparse in cover.
My only thought about the gramma is they grow fast and get large. Generally all show and no bite though there are exceptions.