Short cage derailleur: how to find out maximum sprocket differences?
As a general rule a Short cage derailers should only be used with a single front ring, medium cage with double front rings and a long cage for tripple. 

Naterally this can be effected by the amount of travel your bike has at the back.  If it is a full suss then as the rear wheel moves through it's travel the chain line will get longer, placeing more tension on the rear derailer.