You cannot change/update the SKU at any time when you've creating products using the Viralstyle Fulfillment app. The SKU is what we use to sync your Shopify cart, with our Fulfillment queue.
We are in the process of updating the API so that when importing products it will automatically remove the redundant variants. For now, you can delete the unnecessary fields entirely without any effect SEE HERE.
You are also able to rename Product Variants like Color & Size, but please keep in mind that Viralstyle Fulfillment cannot be held responsible for any updates or changes should they result in customer confusion.