Can I selectively send Push Messages

Yes.
You may send to Apple, Android or all users.

Also, when Subscriptions are enabled, you can send push notifications assigned to one or multiple subscriptions.
For users who have a selected subscriptions turned off on their device, they will not receive the push note alert, however they will still see the message in their Messages tab.
If you choose not to select any subscriptions, the push notification will go out to ALL users, even if they have all subscriptions turned off.