Do you mean non-betta tankmates? A school of 6 pygmy cories is always a good place to start as a tankmate for a single betta. I would also do a school of 6 ember tetras - much smaller than other tetras, very chilled and peaceful and not at all nippy.
If you mean tankmates for the sorority, I would either leave it at the 5 girls or just get 6 pygmy cories, depends how regular your water changes are. I do have a bad tendency to overstock, though, so I may not be the best person to ask on numbers. :p