aymiee glad to see you got everything working. As for this question:
When creating a Sharepoint index, would I need to create the new index in the same Search Service? Or do I add to the existing index (gptkbindex) in the Search Service?
It depends on your scenario. If you want to always ask questions across the content coming from all these sources, and the schema reasonably aligns (maybe with just a few fields specific to each source), it might be a good idea to put everything in a single index. You could keep a filterable field that has the name of the original source, so you can filter which source you want data from, or drop the filter in the query and search across all sources.
I would only do separate indexes if the schema was completely different, or if you want to make sure you never mix content in search results from different sources.