Female betta fish can be kept in a mixed tank, but the following points need to be noted:
Female betta fish can be raised together with other fish, but certain conditions should be taken into account. Bettas are rather aggressive in nature, especially male bettas are prone to fighting with each other and may even get injured or die. Therefore, generally speaking, it is not recommended to keep bettas together with other bettas. However, if you choose some fish that are gentle-tempered, not having a particularly large difference in body size, and swim relatively fast, it is possible to try raising them together with bettas under certain conditions.
Specifically, you can choose fast-swimming livebearers or benthic fish for mixed breeding. Fast-swimming livebearers like mollies, since bettas may attack fish of similar size to themselves, fast-swimming fish can avoid being attacked by bettas. In addition, bettas are surface-dwelling fish, mainly moving around the upper part of the fish tank, while benthic fish live at the bottom. The two won't encroach on each other's space, thus avoiding attacking each other.
In conclusion, female betta fish can be raised together with other fish, but it is necessary to choose the appropriate fish and pay attention to avoiding fights between bettas.