I thought this might remove a few of the initial questions:
nano-reef.com is nearly 22 straight years of logged tanks in a single forum showing pattern outcomes of certain setups. two clownfish in a new tank without any preps is nearly all the jobs on file there, and truly if we scan the years on file it usually turns out ok given decent water care and stability/hardware luck and halfway decent sourcing for the fish. two clowns in a cycled tank tend to do fine if you acclimate them correctly (comes from the disease forum, a hidden detail nobody bothers to verify or they do it wrong {bag floating is bad acclimation} = disease forum touring is more than just disease preps it's also correct acclimation steps discovered)
the problem begins as tanks get larger, they can contain mixed species. when things get above just two clowns and a common small goby, the variation of disease import increases to what you see today in any site's disease forum.
you can see from that initial read thread above, anything wet added from a pet store brings in disease unless someone is going to run a dual system setups (one display tank, and one tank used for fallow treatment of all life to be added)
that's why he discussed building an entire reef without fish first (vs fish in early on) and then letting the full reef go fallow, then adding quarantine-prepped fish.
fallow and quarantine are the two top concepts discussed in the disease forum stickies.
only that careful order would work for disease exclusion preps for a single tank system...buying or attaining qt-prepped fish at the start as most are inclined to do if doing any preps at all is quickly wasted $ when unprepped items like corals or clean up crews are added from pet stores over the coming years as the larger tanks are built up.
you'd have to fallow prep in groups anything wet you ever buy from a pet store, requiring you to run a separate second observation tank if you want disease preps done the right way, unless you choose to reef like that thread above shows with a delayed fish payoff.
the #1 overriding factor is that when new keepers see just how tricky disease management is, they tend to just abandon it and take their chances. to see how that turns out, use the disease forum help threads smartly
take time to go to each help poster on page one, click their avatar
select find all posts
go to their first post, that usually gives a date and a description you can use to determine tank age, related to the date and details of their help post.
notice this trend by page two: nearly all posters broke basic security controls and/or they were dealing in new reefs 6-8 mos old at the time of beginning to lose fish. some will even be clownfish-only setups but the majority are mixed fish systems, full of corals and clean up crews maybe roughly a year in, using no security steps at all and now their whole system is disease infected. predicting that trending can save you time and money depending on how complex of a reef tank you have planned.