Hi Giorgos,
It would be a good idea to delay placing your new Heckels into their final display tank so you can treat them. I'm sure that your source has begun some of the usual treatments but it is best to perform them yourself so you know for sure what has been done.
I usually use a higher temperature in quarantine of wild Discus than what I use for maintenance. I would begin at almost 32C for the first week then lower the temperature to 28.5C or 30C. Try to match the pH and hardness. Hopefully They are already being kept in soft acid water. I would add a black water extract and provide low light levels. This will help them to feel more secure and less stressed.
I use flubendazole, praziquantal(Droncit in Europe), and metroniadazole. I use these per manufacturer's recommended dose except the Flubenol 15 which I use at half strength. keep them in quarantine at least two weeks. The correct time is going to depend a lot on what condition they arrive and how well they are eating.
Later on and once the Discus will eat a Discus granule type prepared food I soak some in Prazi and Flubenol and feed them medicated food once a day for one month in addition to what ever frozen or live foods you are using. The initial medicated baths will do much but it is difficult to achieve therapeutic drug levels inside the fish unless the receive a course of medicated food otherwise they may continue to retain many parasites in the gut.
You could carry out most of these treatments in your large tank but it is always best to avoid it. Medications work best when there are no dissolved organics in the water, the dirt that has accumulated in the substrate and any wood used for decoration.
Less medication will be used in a smaller bare quarantine tank and no other fish and plants will be subjected to any unnecessary chemicals.
I hope your new Discus come in superb condition.