Combining Shimano RS500 crankset with Stages Shimano 105 5800
Many individuals are running non matching hollowtech II stages crank arms with no ill effects.

Stages themselves cites this as OK. 

Note the Dura-Ace crank below.

Crank Compatibility: All Shimano Hollowtech II road cranks

Go ahead and purchase with confidence and enjoy cycling with power.

I run an Ultegra 6800 Stages on a 6700 Crankset personally.