When I was considering female bettas for my tank, someone told me to buy three or more rather than two, in order to avoid aggression. I don't know if adding more would help at this point, but your post made me remember this advice, and I thought I'd mention it in case others can chime in with advice or experiences.
Yes, this is very true. With female bettas, ideally you want to have either one in a tank, or else "a bunch." When you have two, then the scenario described by the original poster is all too common; the dominant one constantly harasses the other one. Whereas if you have a group of them, there are more fish to "spread out" the aggression. They may scuffle with each other for a few days when they are all first put together, but they are only doing that to establish the "pecking order" and then they all settle down with a minimum of any aggression after that.
For the time being, it sounds like you did the right thing by separating them. Depending upon the temperment of the betta with the shrimp, the shrimp may survive--or may be lunch by tomorrow. My guess is if the shrimp can last 24 hours with the betta without getting eaten, then he will be fine.
Another thing about female bettas, they are
notoriously individualistic. It is really hard to make too many generalizations about their behavior because each one tends to be so different from the others. One will gobble up all the flake food in the tank, another will decide she's going to refuse flake food entirely. One will spend 95% of her time swimming just under the surface of the water, another will spend 95% of her time at the substrate pretending she's a cory. One will spend most of her time at the absolutely farthest part of the tank from any current, another will boldly swim in & out of a filter current repeatedly for an hour or more at a time.
And yes...one will decide having tankmates is a perfectly fine thing, while another decides the entire tank is going to be "hers" and any other betta in it is an intruder. :-| While probably 90% or 95% of female bettas can be kept in a tank with a whole group of other female bettas without problems, (and indeed will be very happy and thrive being in a big large tank with company), there is always that 1 in 20 who just seems antisocial and will never be happy unless she is in a tank all by herself--even if that means being alone in a tiny 1 gallon thing. It's just the way they are.
So if you have a tank big enough to do it, you could probably buy another 3-4 more female bettas and maybe they would all get along just fine. Or, you might do that and discover that one pink one you have is just a particularly antisocial one who is going to need to be in a tank all by her lonesome in order to be happy.
Please keep us informed how things progress.
