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No. Well, kinda. Fireteam raven existed, but then meeting up with cheif, various vehicles and weapons were not at the battle of io4
The story told in FTR is canon. They were present at the battle at i04.
Yup, definitely canon :)
But their encounters with chief arent. Things like the falcon and grenade launcher were experimental tech exclusive to reach, and the flood grave mind was a proto mind at that point so pure forms and juggernauts were not possible.
The encounters with Chief are the only ones that are squishy. The rest all work out just fine, and we've actually covered some of them in Canon Fodder issues.
Not a problem at all, it's canon. :)