Socrates - There's no requirement to have an FI rating to sit on the right hand seat. Nor does the seat you're in determine whether you can log PIC (not P1)
However it would be a foolish pilot who leapt into an unfamiliar situation like flying from the right, without trying it with a pilot or instructor on the left first. There is a detailed report on the AAIB web site of a crash involving (I think) a PA28 which stalled and spun in while being flown by an instructor in the RHS. A major factor was the positioning of the ASI which meant that the right-seater didn't have a clear view of it, and the reading could be misread.