I have Madone 4.6c with Rival and love it, but feel the rear derailleur could shift smoother. I'm thinking for about $120 I can purchase a new Sram Force derailleur. So here's a few questions.

1) I assume it's a straight swap... correct?

2) I assume the Force Rear Derailleur will work with the Rival shifters... correct?

3) The cassette is a 11-28 and the current derailleur is medium cage. Do I need to get a medium or will a short case fit the 11-28?

4) If I do get a short cage I assume i'll have to shorten the chain... correct?

Thanks guys.