Most keys will fit without any issue, as long as the hole on your key is greater than 4mm in diameter, it has a flat profile, and the length is shorter than 67mm (from the hole to the end of the key). This rule also applies to keys where the holes are irregular in shape (e.g. slot, square, triangular). For your longest key, we'd recommend to place it in the middle of the stack for a better fit (except for Mortice Keys).
For anything else that doesn't fit on the Orbitkey (such as car keys, key fobs, bicycle lock keys) can be attached using the D-ring included with every Orbitkey.