The hard part is quarantining something that needs more than a bare tank of warm water...structure,plants. If they are larger fish,it means keeping a large extra tank going..not many people are willing to do that when it's going to be empty of fish in a week or two.
But there is no true "safe" way to add new fish to an aquarium of fish months or years established. Nobody has invented "make adding new fish 100% safe and it never bothers resident fish and plants, Solution X".
How are the smaller rainbows in terms of health?A bin would never do for Discus..but yes,they would be among the top two fish to quarantine. Yet,they are so touchy that after the dim lights,no substrate and very clean water of a q- tank, a bright community tank might do them in. Still,its a chance as they are highly prone to contagions.
Large fish on the other hand that look healthy? Pretty much are. Your gladiator central American cichlids, the Pacu family including Red Hooks are nearly impervious to illness.
I could pick and choose many fish I've never seen get sick.. Leporinus is another,Pleco's- large types..same. All will live a decade (give or take) easy in any average aquarium.
Oooooh no is this true??"Bacteria is free, yay!"
It most certainly is, but people continue to waste their $ on the bottled stuff (often at the suggestion of LFS employees, who are there to make a buck, and usually don't have a clue about fishkeeping)