In a fanfiction, Naruto's powers are based on chakra, which is different from the magic in Fairy Tail. His ability to quickly learn new techniques and his huge chakra reserves would make him a force to be reckoned with. Itachi's Sharingan has so many abilities like seeing through illusions, predicting movements, and using powerful genjutsu. The Fairy Tail members would be in awe of these powers. They might even try to find ways to incorporate some of their aspects into their own magic, but realize that Naruto and Itachi's powers are very much their own and difficult to replicate.
Naruto's power would be seen as something very energetic and physical. His Shadow Clone Technique would be quite a spectacle in Fairy Tail. Itachi's power, on the other hand, is more about control and deception. His Mangekyou Sharingan would be regarded as something very dangerous. The Fairy Tail guild would probably be initially hesitant to fully trust them because of the sheer power they possess. However, as they see Naruto and Itachi using their powers for the good of the guild, they would start to accept and even admire them.
Naruto's powers, like the Rasengan and his ability to summon toads, would be seen as very unique. The Fairy Tail wizards would be both amazed and a bit confused at first. Itachi's Sharingan would be considered extremely powerful and perhaps a bit terrifying. His genjutsu abilities might be mistaken for some kind of dark magic at first. But once they understand that Naruto and Itachi are on their side, they would be respected for their powers.