Okay, to add to this question: I am day, night, and IFR current in multi-engine aircraft. If I want to be truly legal to take passengers in a single engine aircraft which I haven't flown in over six months I can take the single engine out at night and get three stops to a full landing and it will requalify me for both day and night passenger flying in the single engine. Common sense tells me I will fly it more to regain proficiency and I know it has to be truly night.

What I can't remember is if my IFR currency overlaps between multi and single or do I need to go get instrument proficient in a single engine as well?

If your instrument rating was issued to your pilot certificate as "Instrument Airplane" you're good to go in either provided you also meet the recency of experience to act as PIC for both your pilot certificate and the instrument rating. The rating is category based, not class based.