Yes, you can access the Shipment Tracking - Unified API for all shipments, regardless if they were created via SOAP or REST.
Example given:
You are a user who is creating shipments via SOAP for DHL Express and DHL Freight, but also via REST for DHL Parcel. You can track all those shipments via the DHL Shipment Tracking - Unified API.
The easiest and fastest way to see if a shipment will appear in the API is to track it via our https://www.dhl.com/ site since it uses the Shipment Tracking - Unified API as a base.