In reference design special shape (narrow oblong instead of round) of a 1st row pads is applied. 

How often this trick is used? What is about soldering result for a volume production?

BGA has 0.46 pitch, without this trick 3 mils width/separation is required, with it 3.7 mils width/separation is acceptable.

These traces and pads are most complicated place in particular PCB design and defines fabrication requirements for a whole project.

Default pad size is a 10mils, oblong pad has 7 mils x 13 mils dimensions.

How it can be calculated for proper soldering? 

Is it possible to change size even more (6 mils x 14 mils) to get low-cost 4 mils width/separation requirement for fabrication?

Many thanks!