Actually the answer to your specific question is no. Apple will support some (limited) domains (Messaging, Lists, and Notes). Music streaming is not one of them.
SiriKit on HomePod
With the intelligence of Siri, users control HomePod through natural voice interaction and can conveniently access iOS apps that support SiriKit Messaging, Lists, and Notes. Siri recognizes SiriKit requests made on HomePod and sends those requests to the user’s iOS device for processing. To prepare your app, make sure that your SiriKit integration is up to date and that you’ve adopted all of the appropriate intents.
Test the voice-only experience by using Siri through headphones connected to your iOS device with iOS 11.2 or later.
Does the Apple HomePod speaker system support additional functionality through third-party apps?
Yes. According to Apple, iOS 11.2 enables SiriKit for HomePod functionality.
iOS 11.2 introduces SiriKit for HomePod, the powerful speaker that sounds amazing, adapts to wherever it’s playing, and provides instant access to Apple Music. HomePod is also a helpful home assistant for everyday questions and tasks. With the intelligence of Siri, users control HomePod through natural voice interaction. And with SiriKit, users can access iOS apps for Messaging, Lists, and Notes. Make sure your SiriKit integration is up to date and test your app’s voice-only experience today.
will I be able to stream services such as SomaFM through a HomePod unit?
This will depend on if SomaFM implements the feature. However, if you can play SomaFM via Siri, then you should be able to stream it as well.