I did a google search for short cage derailleur advantages and found this. It sounds good and makes sense to me.
All the derailleur cage does is take up slack in the chain when smaller rings/cogs are in use. It follows that:
Short cage= Common on bikes with a narrower range of gears, especially road doubles. Allegedly better shifting (not really true). The ability to run a shorter chain, which means less chain slap on a mountain bike, and generally more tension on the chain. Better ground clearance (off road), but most derailleurs die from catching sticks, not hitting the ground. Lighter by a very tiny bit. Looks cool to some people.
Long cage= Common on road triples and almost all mountain bikes. Can handle a longer chain, which means that it can handle a larger "chain wrap." This means you can have a wider range of gears without experiencing slack in the chain, or having to use a chain that is too short for some gear combinations.
You should be able to use a mega 9 derailleur on an 8 sp setup.
I think you can use your current shifters but the shifting might be backwards. If you shift up with your thumb, now you may shift down with your thumb. Or something like that.