Neji saw the red chakra but didn't know it was from the Fox. I think some high leveled ninjas can sense chakra and people intents from others. Like Kakashi knowing that Gaara was on the other side of the rock. Sasuke didn't know but he did. I've only notice advance ninjas ever saying anything. Plus most can tell different chakras from another right? And if someone knew that he had Kyuubi in him and he was calling upon a completely different chakra its obvious.

As for everyone being amazed... just because like Neji-Aniki-sama said it amazed them that he was able to produce chakra after what Neji did.