If you replace the guages, yes they will apperar in a new terrain. However, at the moment, all planes use the same set of guages. If you replace with metric then everyone has to use metric - which will obviously be hell for people used to imperial....
Also, looking at your variometer: while larger bmps may work for some pople, Htc have said that they will not allow them in official terrains as not all graphics cards will support them.
This is something close to my heart as well, i'd love to see support for plane-specific guages.