Just a quick update: The betta/platy combo was working out fine until today. I walked up to the tank to see the platy bite a big chunk out of the betta's tail
Then I noticed that the end of his tail fin and bottom fin had both been eaten at. I scooped the platy out and put him in my husband's tank with the other platies. Then I did a 50% water change in the betta tank. I'll keep doing water changes for the next several days, and hopefully I'll be able to help the betta avoid fin rot or other infections.