Tee Emm, not really sure what your geographical situation is (apologies if teaching and sucking eggs come to mind please) given you talk of Australian CASA but your profile says you're in the UK, however under JARs (and their EASA equivalent when they get around to licensing I guess) then the instrument rating confers only single pilot or multi pilot privelages and must be maintained separately. The multi pilot instrument ratings obviously require a type rating to go with them, hence the IR is only valid on that type. The single pilot IR is split into single engine and multi engine, the latter allowing you to exercise the privelages on the former as well if I recall correctly. So all that remains is your class rating be valid and you can exercise the privelages of your single pilot IR as you so chose, with the caveat you must be able to demonstrate your ability to manage the flight and control the aircraft without flight directors etc whilst an engine is failed, and god forbid do the radios
If any of the fancy kit is fitted to a GA aircraft you sit the test on though, prudently, the examiner will check you know how to operate it as well. Is this broadly what you're pushing for?