Hello! With regards to adding tankmates, it really depends on the betta's personality. Some bettas are relatively peaceful and won't mind some friends, while other bettas can't stand anyone but themselves. Though females tend to be less aggressive, there's always a chance you could have a feisty little girl.
In short, be prepared to return the other fish in case she decides she doesn't like her new roommates. The best way to do it is to put the other fish in the tank first, and then add the betta. If you do it the other way around, the betta may see them as intruders, so adding the other fish first helps her get used to them as part of her new environment.
Some non-nippy schooling fish could do well, like White Cloud Mountain minnows, harlequin rasboras, ember tetras, or neon tetras. Make sure to have a group of at least six.
You could also do a small school of Corydoras. Since they are bottom dwellers, they'll have a higher chance of success as they stay out of the betta's way. They're also some of my favorite fish and very adorable. There are tons of colors and patterns. Again, these need to be in a school of at least six.
Snails might also work, like a mystery snail or a horde of ramshorns (just be aware they breed prolifically if provided lots of excess food).
Hope this helps
