i have a pregnant platy and two 20gal (normal) tanks. I am waiting for the platy to give birth to put the babies in my second 20gal. My question is: can a female betta and some platys coexist? I would put the betta in when the platies were at least two months old
Honestly, it depends on the Betta. The two can coexist temperature wise and stuff, but the Betta´s aggression may be too mcuh for the platies. Try, it out. Dont put small platies though. If it works and the Betta is not overly aggressive then yes they could coexist. Sometimes it works, other times it doesnt. Depends on the Betta´s tolerance.
You can do what I call a drop test, where you take a fish with the betta and how you would acclimate it. If the betta has interest in it, it is more likely going to become more interested as time progresses and eventually may hurt the platy. Or if it has no interest during the acclimation process, I would just leave your fish in the tank and watch for any signs of distress or markings on it for the first two days.