Use a metal wire keycap puller, not the plastic kind. The spec for keyboard spacing defines the space around each key, so on most keyboards the puller has the margin for the key you're pulling and the margin for the key above/below. So yes, a 1u cap by itself is a tighter fit. The metal wire pullers work just fine.
Also - Yes, we've been tinkering with aluminum. Due to the multi-faced shape with interior gaps, milling is problematic to say the least. 3D printing works but the price is out of control, we're watching for metal 3D print quality improvements and mass production price improvements.