Query

I am thinking of purchasing the Canon 400mm f/2.8, is this too long for field sports like rugby or soccer?

Depends where you are on the pitch :)
If you're stuck at one end and want action the other end, then I'd say no it's not too long.
Assuming a full frame body that is.
If it's an APS-C sensor then it's different.

I think I would want many bodies and lenses all at once :)
10-22 or so behind the goal for soccer.
24-70 for close action in my half.
70-200 for action middle of my half.
400mm for action in the other half.

Depending on how close I could get to the pitch, that would vary of course.
Some matches/pitches you're not allowed close enough for the 24-70 to be useful.

Take a look at the photographers at the 6 nations, or on MOTD.
They're using fairly long white lenses on monopods, then switching to a black lens closer up.