Monoflo

I have a small commercial building that I am putting in a new cast iron boiler.  It is an old monoflo system with a two inch steel pipe main and 3/4 branches to registers.  (Fin tube cabinet type).  It also has an add on in the back of the building with a one pipe 1 inch line... The supply house designers said the two inch line is over sized and decreased the boiler loop to 1-1/2" and the zone comes off 1-1/4 to the supply to the two inch monoflo side of the system.  Just wondering if this will be adequate.