Does the ability to have gold/silver sexed link chicks come from his RIR or SS side? Or does it come from the hens?
All of the above. To get this kind of sexlink chicks, you need a rooster with two gold genes (one inherited from each parent) and a hen with the silver gene. You could use a pure Rhode Island Red rooster or a pure Speckled Sussex rooster and either would work just as well as your mixed rooster.
A rooster has sex chromosomes ZZ. To be the father of sexlink chicks, he needs to have the gold gene on both of his Z chromosomes. He inherited one Z (with the gold gene) from each parent, and will give one to each chick he sires.
A hen has sex chromosomes ZW. She is either silver or gold but not both because she only has one Z chromosome. She gives the Z chromosome to her sons and a W to her daughters.
When a hen is silver, she gives that Z chromosome to her sons. Because silver is dominant over gold, the sons show silver,* even if they inherited gold from their father. The daughters inherit W from their mother to make them female, and a Z chromosome from their father. So the daughters of a gold rooster will always be gold, because they have gold on their only Z chromosome.
*Note, sons that have one gold and one silver gene look silver as chicks, but often show some amount of yellow or red as they grow up. So as adults they do not quite look like pure gold or pure silver. But as chicks, they just show the silver.