Unlikely, insider builds are supposed to be at least one step ahead... even on slow ring..
However, you can get on the Insider program to get Windows 10, then un-enroll from the program. That will stop you from getting the future builds, and you will then only update when the official Windows 10 updates. In this way, you can bypass the carrier and keep the "stable" version of Windows 10 on your phone.
And the 635 isn't one of the least powerful Windows Phones out there by a long shot. There are many Snapdragon 200 models that I wouldn't want to put it on, but that is supported as well.
The 635 is actually the most powerful of the 512MB devices, so if any 512MB device will run it, it will be this one. It's got the same processor and CPU/GPU speed as the Lumia 830 and Lumia 640/640XL, just less memory. Since Lumia 640 is on the front line to receive Windows 10 first, it's not the power, but the memory the 635 should be concerned about. However, with Microsoft's assurances that it will run well on 512MB devices, I don't see the issue.