Hi Szimi,
Axios, Loudias and Aliakmon Delta is a huge area. Permission is not needed to ender the area. You can contact the Management Authority of the park at
wicaxios@otenet.gr tel 0030 2310794811. They based in the town of Chalastra. You can enter the eastern part of the area from that town. You can check also the lagoons of Kalochori (the most eastern part of the Delta) for terns and waders.
Other interesting sites are Alyki Kitros (as you mention), and the small lagoons east of Thessaloniki (few Kilometers from the airport), lagoon of Epanomi and Alyki (salt works) of Agelochori (from the names of the nearby villages). If you have time, you can visit also our area (lake Kerkini) that is 1,5 hours driving from Thessaloniki. You can check the following address
www.lake-kerkini.earthlife.net for info about the area.
If you finally decide to come to Kerkini let me know it by send me an e-mail (or if you need more info for the area).
Theodoros Naziridis