I remember Guinan was announced as being one of the characters to appear on DS9 when it first premiered but it was confirmed by producers that Whoopi's schedule was too busy.
Depends how you define "plausible". A bunch of other TNG characters turned up in the first season for one reason or another.
I presume Guinan can leave the Enterprise for a period, just like everyone else does. She is a curious traveller who may conceivably want to see the famous wormhole. It draws visitors from across the quadrant, so why not her?
She's not even seen during TNG Season 7, so popping up on the station for Rivals would have been no continuity issue.
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