I doubt that, I think all pokemon that were exclusive in Silver will be exlusive in Soul, and all Gold exclusive will be exclusive in Heart, Hoppip and Mareep were catchable in both versions.
They are making A GSDS reamkes!!
Means more Hoppip and Mareep(Woohoo)
But i have a feeling that They are on a deffrent version,As i mean Mareep in SoulSilver and Hoppip In HeartGold,or maybe the opposite.