I might consider cory's such as Sterbai corydora's, or Peppered corydora's that do better than other's at tropical temp's that betta's enjoy.
Many of the corydora's don't much care for water warmer than 75 or 76 degree's F in my opinion/expierience.
Would let the tank age a bit (couple month's) before introducing the cory's.
This will ensure there is stable parameter's,and mature substrate for the cory's to root around in.
All kind's of little critter's establish themselves over time, near the bottom,and mulm that should be present, provides a good place for these fish to search out food's.
The corydora's sometimes don't do well in Newly established tank's, and often do better in tank's with some age.
Just my two cent's.