When I say that I'm not saying that you don't know how to care for fish. There are things I do that I feel bad about. Like keeping my fish in a higher PH than I should... But that us kinda out of my control at the moment.The point I'm making is that when I think about it, 8 3-4 inch fish in a 10g is just too much. Seems and feels like torture...
I feel like if you were breeding betta fish the minimum tank size should be 10g for a pair. As an ABSOLUTE MINIMUM. Now I've never bred them and don't plan on it but I just could not see putting 8 in a 10g unless they were growing fry.
You also said that the minimum tank size for one betta is 2.5g and the reason why they say you can do that is the oxygen. Since bettas can breathe through their labyrinth organ, along with other fish like gouramis (Osphronemidae), they don't need large spaces FOR OXYGEN. But I also feel that is not the only thing to take into consideration. What about exploration spaces and room to really move around in?
I would say for one betta 5g is absolutely the lowest gallon size to go. Any less and when I see them in there it makes me feel sad, even them in a 5g makes me feel sad.
I had one betta before in a 3.5g... Now I didn't set the tank up right at all but when I looked at his size compared to the tank itself. I felt bad for setting it up.