FW, at the `fix`/step,the a/c flies at an indicated altitude of 5000ft.Another a/c is vectored to 4000 ft in the same area.
The higher a/c is at a true altiude of 4700 ft under the conditions.
If it hit the fix at 5,000’ which was also the charted value, then conditions are ISA, so it is at a real 5,000'. As soon as they vary from ISA, the indicated crossing altitude will differ: less in warm conditions and more in cold conditions.
As the atmosphere warms and cools, the “feet” that your altimeter indicates also expand and contract and are only equal to the reference foot in ISA conditions. An ILS glide slope, however, follows the same path through the sky irrespective of temperature. It would do the same even if there was no atmosphere at all.
The fix crossing altitude stays the same in absolute terms, as it is defined as a distance from the glide slope transmitter. What you see on your altimeter as you cross this fix depends on the composition of the atmosphere below.
In terms of separation from other aircraft, when you are on the ILS you are following a fixed path no matter whether it’s Monday or Thursday or if it’s 40degs or -40degs or if you have 950hPa or 1050hPa on the sub scale. Other aircraft who are using their altimeters on a common pressure setting will be separated from each other but not necessarily from you.