Window 10 for Phones first preview will not come to all the devices is now well-known, though more devices will get added as the preview progresses to newer builds. It is also good to finally get reconfirmed that all the Windows Phone 8 devices will ultimately receive Windows 10.
Now, one of our regular sources has been kind enough to let us know about a small list of devices he knows are being used to test Windows 10 for Phone in insider circles. Though he emphasized that this list is not comprehensive and Microsoft may have more or less number of devices, when the preview finally goes public.
Lumia 635 / Lumia 630
HTC One M8