Quarantining fish is not simply about keeping diseases out carried by the new fish. It's also about getting new fish used to what you are putting them into.
Your older fish may have built up a tolerance for whatever bacteria the water they have been living in, or the parameters they are used to.
A not so common Placidochromis prefers hard alkaline water, and although the others in the tank may also be rift lake species, they may be more aquarium strain than the Placidochromis, and although your LFS says your water parameters are generally fine they may be more acidic, or have a drastic osmotic difference than the waters the Placid was raised in, or used to, that along with the stress of shipment, and new habitat, been may be more than it could handle.
There may be semi non-pathogenic bacteria established in your tank that your present stock has grown immune to, yet can infect a new comer that has no tolerance for.
You can often drop a managuense or lemon yellow into any tank and it has no problem, but take one of the sensitive Theraps or Enantiopus, do the same, and find them dead or diseased the next day.
There are many variables to fish health, and the more you get into less common species, the more care must be taken when introducing them to a tank.
I would always quarantine a new fish for at least 2 months, gradually adding water from the tank it would go into, to prep it for what was in store. I know a little anal, but the more you get away from bread and butter species, the more expensive they get, so extra care is called for.
