Not quite true re IR validity. A single engine IR under every licencing system I know is only ever valid for SE aircraft**. A multi IR, however, is valid for singles and multis.
Tinstaafl, in the USA, the instrument ratings pertain specifically to a certain category of aircraft, Airplane, Helicopter, Poweredlift. An instrument airplane rating is valid for both Multiengine and Single engine on an FAA ticket, however to fly instruments on a multiengine airplane, one must also have a multiengine rating and an instrument rating, but in the US, there are no ¨multiengine instrument¨ and ¨singleengine instrument¨ ratings.