Since the announcement of Halo Legends, many people have voiced concern that the anime will clash with current canon. There have already been many problems seen in the trailer, including a slender Spartan and a gundam like mech suit. These problems would appear to be a disaster for Halo Nation.
There is a saving grace. Due to the name of the anime, "Legends", there is a possible explanation for the mistakes that would be made; the title is more meaningful than one would first realize. The title may actually mean that these stories ARE legends. Each story could be told by a witness, which would mean that embellishment would be made by the story teller. Each of these discrepancies would be explained by error in memory and other similar reasons. For example, the silly short with Spartan 1337 would be told by and ODST. The ODSTs hate the Spartans and that would explain why that spartan appears so stupid.
I believe that Legends will work in a similar fashion as the believe campaign. Each of these stories will be interviews that were recorded years or even decades after the event. Therefore, the stories would still be considered to be compliant with current canon.