Dear Dr Gargeshwari,
Since I see you to be in favor of sound research methodology, let me tell you that when someone asks for a reference for a certain story/narrative involving the characters from a well known or well established mythological epic poem, we need not comment on the justifiability of the events in the narrative being searched for.
Various folk-versions of mythologies can not be just brushed aside as one can add any story as one pleases in mythology. Folk versions of mythologies are very important area of study that requires and is already being subjected to a systematic methodological study.
Coming to the version of Ravana being a cursed Vishnu Paarishada, it may be noted that that version is not from Valmiki Ramayana and it is from the Bhakti /Bhaagavata versions of Ramayana. The cursing characters in that version are Sanaka, Sanandana, Sanatkumaara , Sanatsujaateeya. There could be a version in which they are mentioned as Saptarshis which I do not know.
The version mentioned by aadaraNIya Sri Hari Paarshada Dasji is an interesting, obviously, folk version. Many folk versions of these epic narratives have weird-looking aspects. But for that reason they need not be brushed aside.