It doesn't matter if the paths of pain is an ability or not...the fact remains the same he jumped Jirayia and Kakashi that wasn't fair...Itachi will own Nagato...
It is an ambush which is entirely fair....they are shinobi, and as such stealth is to be employed.
Is Naruto using shadow clones to increase his combat cheating?! If you think it is then I csn understand why you consider it cheating and so I will argue with just Nagato on his own.
Nagato on his own has 6 primary path abilities in addition to any elemental tech he may have.
1. Deva Path. Shinra tensei pushes away any attack. As such, Totsuka blade cannot harm him.
2. Preta path. Absorbs Jutsu. Amaterasu cannot work. Also provides a way to drain his chakra in a lock.
3. Animal path. With all of his summons that adds a variety of creatures to engage Itachi. At their worst they waste his chakra as Amaterasu shields. Could also hide in order to act as eyes for a battlefield view (chameleon or centipedes).
4. Human Path. If he gains a grip with Itachi, he can simply take his soul (can combine with Asura path to grab him).
5. Asura Path. Mechanized weapons from missiles to lasers. Csn sprout additional limbs for a grapple.
6. Naraka path. Another optipn in the grapple. Csn take his spirit this way. Can be used to revive fallen animals (can use shinra tensei to remove Amaterasu's flames).
Nagato's chakra is far above Itachi's who actually has an extremely weak chakra reserve. As such, Nagato simply has to out las Itachi, which Shinra tensei combined with his summons and other paths allow.
Chibaku Tensei alone csn defeat Itachi as it required Yata Magatama (spiral tomoe disk), Rasenshuriken, and a Bijudama to break it.
Itachi would not "stomp" Nagato...that is disrespectful to Nagato's character. At the same time, Nagato would not "stomp" Itachi for the same reason. Both are talented, but Nagato has the stamina and tech advantages.
Edit: To the counter argument that Amaterasu hit Nagato I respond as follows: Nagato still had degrees of self control when he was hit. How do we know? Edo tensei's orders were to react to enemy Jutsu. Nagato later, after losing full control, instantly uses Shinra tensei. A clear sign that he was simply sitting there while hoping the flames would burn him away.