If you click on the highlighted name Guppy in your post and read the profile, you will learn that guppies and bettas do not make good tank mates. Also, bettas and bettas do not make good tank mates. This is probably why they are all so mean to each other.
I'm sorry for vanishing. I suddenly got a lot of work in to do. The majority of what I've read is about male guppies and male bettas being a very bad thing. I have come across posts where people have female bettas with guppies though.
Your female bettas each need their own house. Whether they are actively nipping each other is not the only indication of bullying and stress - they release hormones into the water that are signs of aggression. Also, they tend to be more active at night, when you are probably not watching them. Right now the guppy might be mean enough for all of them, but removing the guppy won't eliminate the betta-betta problem.
I took the guppy out for about 4hrs. for a "time out." I had never done that before but I've read about it many times on various forums. People give the fish a time out, rearrange the tank and add the bully back in. Sometimes that's all that's needed to settle the little terror down. To my surprise it did help the guppy. I've read about the hormones on cichlid forums.
Guppies are aggressive with certain other fish, and bettas are on that list. Take both bettas out the tank and into their own tanks, and put non-colorful, short-finned community fish in with your guppies. Or even just more guppies.
At this point the tank seems so established that I'm going to leave it as is. Everyone is getting along well. The 2 bettas swim together. No flaring, biting or anything negative. I think I may have that rare instance of 2 females actually getting along. I actually had this same thing happen years ago. I didn't know that you're not supposed to house 2 females together so I did. They were best buddies. It's hard to believe I'd get that lucky twice. Of course if things change one will be removed.
With cichlids, if you introduce them when young there is a better chance they might be able to grow up together and live in peace. I wonder if this might be going on with the bettas. Of course sometimes all the young ones get along "until" they mature and then all heck breaks loose.
None of the fish you currently have in that tank are compatible with each other except the 2 guppies (assuming they are both the same gender). If the fish store told you this was OK, they were wrong (they usually are).
I don't listen to fish stores. I know better.
I hope that didn't sound too mean. If there is more information that I don't have that makes my advice incorrect, I apologize, but I really do think you need to rethink your tank stocking.
You don't sound a bit mean. Just concerned. I'll let you guys know if things change and I have to seperate anyone.