As indicated above, couchbase lite/ Sync Gateway only supports default scope and collection at this time. So it will be synced to the default scope (and _default collection) on server side.
You mentioned 120 buckets. Do all buckets need to be synced to a mobile client? I suspect not. As suggested, If you consolidate the bucket into scopes (and collections), you can keep the “Mobile aware” data in _default scope.
With the _default scope, the typical pattern for data segregation is to use a “type” identifier in documents and associate each type of document with a sync gateway channel.
In future, when sync gateway and couchbase lite is collection-aware, you can migrate mobile documents into user defined scopes.