When drawing from a 1.5" and returing via a 2" pipe, what happens is that the GPM is the same in both pipes but the water velocity (ft/sec) in the 1.5" is faster then the 2" pipe.

The only real issue with this type of plumbing design is head loss. This reduces your GPM and increases the time it takes to turnover the pool which means that your pump needs to run longer and your electric bill will be higher.

I would recommend 2" piping as a minimum and I would even go up to 2.5" on the spa depending on the number of jets and the HP of the pump.

In order to make better recommendations, I need:

Pool volume
Pad to pool distance
Number Spa Jets and type
Number of skimmers, main drain (if more than 1 will there be one or more pipes to the pad?)
Number of pool returns (if more than 1 will there be one or more pipes to the pad?)
Heater? yes/no
Pool vacuum or other water feature? Give details
Pool builder suggested pump size