I imagine one of more of the mobile guys will be able to give a bit more of an in depth answer, but this is the part where you’ll start having issues:
I would also have multiple database entries for each service all pointing to the same bucket in my sync-gateway config file.
Sync Gateway expects one bucket per database. The thing is, the bucket isn’t just used to contain the exact same documents you put into Sync Gateway, it’s also got user information stored in it. There’s a bit of discussion about it in this thread that probably explains it better than I can.
Assuming for a second that the backend stuff would all be going through Sync Gateway, using channels as you mentioned definitely seems the way to go. If you were thinking of doing something read-only with the SDKs, well… in theory the same data that’s available to Sync Gateway is all there in the bucket, including what channels a document is in…