You're right, there's no rules that say one can't lop off limbs in a photo. In fact, really tight framings in my opinion are done only by highly skilled photographers, who know exactly where to focus and get the action really sharp. Loose framing on the other hand, isn't wrong either, but you do notice you have a few shots that show only the backs of the players, along with a messy background. I'd rather use the 300/4 without the TC or just the 70-200/2.8 alone, to throw the background out of focus as much as possible, with the aperture really wide open. Next thing is probably trying to crop the photos that look balanced left-right, without dead spaces offsetting the photo too heavily on one side. Of course, these are just guidelines, not rules per se.
You do have one good shot - the one of the keeper diving (I think?). Speaking of tight framing, I believe you experienced how hard it was to frame the way you did, and still get the image sharp. Imagine framing the shot through the viewfinder EVEN tighter - which I am still learning. Tough!
But that aside, probably it's a good idea to shoot with the lenses without the TCs - because adding a TC will only serve to slow down your AF response.
Edit: try these links
http://www.sportsshooter.com/news/673
http://www.sportsshooter.com/news/672
http://www.sportsshooter.com/news/671
http://www.sportsshooter.com/news/670
http://www.sportsshooter.com/news/669