you need to have a minimum amount of hours on the type of aircraft to do your IR skills test (ie- single or multi). i think its 55 hours but cant remember.
if you do an IR on a multi engine aircraft, you will get a multi engine IR and be allowed to fly Single engine aircrafts as well under IFR rules. if you get a single engine IR you will only be allowed to fly IFR on a single engine aircrafts.