Questions about Solder Mask Expansion Rules and Passive design in Altium
Use the IPC wizard to design the footprints from the device dimensions and tolerances. QFN and discretes are definitely covered, DFN may not be, at least in older versions. 

You should take into account the registration and minimum solder mask sliver your PCB house can handle, but normally 75um and a 75um sliver is okay. 50um seems rather tight. For pitches finer than 500um (as with your 400um fine pitch chip) you may need to forgo the mask slivers between lands.