Also keep an eye on the ghost, some may be aggressive and slash at the Betta's fins even if the Betta can't see them.
+1 to this. I just had to remove a ghost shrimp from my betta tank after they had been together for over a month with few issues (sometimes during feeding the shrimp would grab at the betta's fins trying to get to food). Came in one day after work and Cleatus (the betta) had shredded fins all over the place. I looked all around in the tank and the filter to see if he had gotten caught somewhere but all I found was a shrimp with a full tummy and not a shred of fin anywhere in the tank. Since I have removed the evil shrimp and exiled it to the spare snail tank Cleatus is healing nicely. I felt absolutely horrible about his poor fins.