The first batch of fish you get do not need to be quarantined in a different tank. Once cycled, this newly set up tank acts as a quarantine tank. Once these first fish are determined to be healthy in this tank, you may want to separately quarantine any additional fish to be added at a later date.
I think you can have a dozen of either the neon tetras or green rasboras. Also consider the green neon tetra as an alternate choice for the regular neon tetras. They're a little bit smaller than the regular neon and have great color.
No need to quarantine the first fish put in the aquarium.
Yep, medication is used to treat symptoms not as a prophylactic. It always screws with biological processes and water quality. Throwing stuff at tank just in case you might get something is a fools errand unless your doing it in a bare bottom quarantine tank with a well established filter and a well thought out filtration scheme so you can change water effectively, add extra aeration if needed and add substantial bags of carbon etc to tank when needed.
Not all prophylactic medications will "screw with biological processes and water quality". Some will- for sure ( like some antibiotics). But, there are many other meds that one can use prophylactically that will not be harmful to biofilter, fish, plants...
Thats the thing, many lump "medications" as inherently bad in all cases without understanding their active ingredients and what they do.
For instance, I use Paraguard that contains malachite green and acriflavine for the following reasons: external parasite control, the healing of surface abrasions, scale damage, and fin damage from transport and handling. I have been using these active ingredients for a decade in the display aquarium with absolutely no disruption to bio-filter, injury to fish, water quality issues. I have very sensitive fish as well: wild discus, altum angels, African tetras.
Ive also used metronidazole for wild discus and geophagus because wild specimens are prone to Spironucleus. By the time you are aware of its presence, It is often too late to bring them back. Metronidazole is harmless to the bio-filter, nor does it lead to bad water quality, Ive used this for 20 years.
All my wild fish get de-wormed with Praziquantel. It is a mild Anthelmintic that in one dose will eliminate gut worms that can go undetected for months with little to no symptoms. These worms are extremely common in wild fish. The 1 dose of praziquantel eliminates the worms and is absolutely harmless to biofilter and water quality.
If your going to use prophylactic meds, know what these active ingredients are, the fish that are prone to whatever this active ingredient targets, the proper dosage, and their impact in the aquarium.