After years of Nikon, just got (and love!) my 7D-- Couple of questions...

Schoolteacher here and very very amateur photographer... just made the jump after a few years with Nikon and got my 7d last week.

I had to rush to get a decent tele, as football season just started... so all I have is a 70-200 f4, and a 40mm 2.8 pancake.

Anyway, shot my first shots with the 7d yesterday, and I love it!

As a former Nikon man, I do have a couple of questions:

1. everyone seems to suggest shooting the Canon in AV mode so I did... even though I used to always shoot the Nikon in shutter priority. I'm happy with the shots, but am seeing a lot of variation in the speeds it selected... some 1/2500 and then 2 mins later, 1/400, despite their being plenty of light... any ideas why? I shot using partial metering... could that be the problem?

2. Do you think I should keep the pancake, which I love but is too slow for our gym, or should I replace it with a 50mm 1.4 to shoot volleyball and basketball? Other way to go would be to get the 85mm 1.8... I have court-side access of course, but our gym is insanely dark, and basically the wall on one side is 10" from the sideline. Which way should I go?

3. Lastly, would appreciate any feedback on my first 7d shots... anything I should be doing differently? I have no ego with this stuff, and just want to take great shots to give to my students...

First 7d photos:
http://www.flickr.com/photos/tbaphotoalbum/sets/72157631459111368/

Flickr stream with mainly older Nikon stuff:
http://www.flickr.com/photos/tbaphotoalbum/

Thanks!

R