Rear Derailleur Question

Then I'll add: Be carefull with your chain sizing. A rear derailer has an additional purpose on most FS bikes, and that is to take up slack as the suspension cycles. On single-ring bikes you can pretty much use any cassette and even though there's chain growth, it's not really enough with one ring to add up to much. When you have two rings, things change, when you have chain-growth through the suspension, you should be carefull. Make sure you measure your chain and cycle the suspension (with shock or spring off). It's not even "cross-chaining" so much that is bad (riding in the 1st rear gear and middle front is always pretty x-chained, but not as much as being in the big ring in that combo obviously), you want to avoid the big-big combo because of how much chain it takes to wrap around the gears. Most people don't ride this combo, but if you found yourself there by mistake and you didn't have enough chain because you're using the short-cage, the results could be catastrophic on a drop or something (chain failure or rip the derailer off).

Going with a short-cage derailer always seems to be the hip thing to do, but many people never think about the above information/limitations. That doesn't mean it won't work, but be carefull.