Pad and soldermask dimensions in PCB design
Usually solder mask expansion of 2-4 mils is appropriate, but the minimum really depends on the process used. 

Silk screen solder mask would be the crudest.  

LPI is intermediate. 

Laser direct imaging of LPI might allow the solder mask to have zero expansion. 

Similarly, the minimum solder mask sliver will depend on the process and the PCB manufacturer, so check their design rules. This comes into play for the sliver of solder mask between two pins of a package with small lead pitch. In some cases you can't have solder mask between the pins. 

For IC lead dimensions to pad dimensions, it's best to follow the manufacturer's recommendation for that package. Sometimes it's not on the datasheet and you have to hunt it down in other documents. My second choice after that would be to use another manufacturer's recommendations for the same package, unless you have something like Altium's IPC footprint wizard which asks for various dimensions and other information and uses IPC recommendations to create pad dimensions.