It's a fair question. I have no experience having them together and I personally will never put them together to test it. I have researched a lot and being on this forum and reading what other peoples experiences have been has educated me further. Sometimes it just works like it's working with you, but some people aren't that lucky. If you have a bigger tank, I think you said you have a 50G? you've probably spread out the aggression and if it's heavily planted, I'm sure that helps too. Like I said, if you are an experienced betta owner and know what to look for (as in aggression etc) than you have a better chance than an inexperienced owner who just want the fish because they look pretty. If you somehow can balance out the aggression, than there's a good chance you won't have problems. But, what do I know. :P Just thought I'd voice my opinion...
And I really appreciate that you're writing what you are writing.
For the fact, I had another setup with one old male and one young, and some girls in a 20gallon tank, and that were no problems there either. And none of my steups are what i would call heavily planted, but there are enough for them to hide and feel safe if they would need to. And I don't really think there is that much aggresion to spread out.
This is an interesting topic.
Since betta splendens are kept for their finnage, I would not advise that they are kept together in one tank.
Yes, many aren't that aggressive, specially older ones and won't chase the life out of others. But they will eventually nip their neighbor (when they want to mate or something). Thus you will end up with torn fins. I would though, keep one male to many females.
Another reason I wouldn't put males in the same tank is that bettas with an attitude (IMO) are better than submissive individuals. They don't stress easily. By keeping them in individual tanks, they should remain aggressive.
If you don't move your fry, don't do 100% water change, don't redecorate, don't disturb in any way, etc, they can be kept together for a longer time (usually until they want to spawn). And if there's an older betta (non aggressive) in there, the fry tends to be more gentle. But they will eventually bite each other.
By comparing experiences with local friends, I conclude that there is a genetic influence on behavior. My females never harm the male in any way, But my friend's always tears the male's fins. This behavior was passed on for generations (about less than 2 years of their mass production and my monthly breeding).
So, if one betta in one certain area shows a gentle behavior, it is likely that many others are as gentle. Since one area is usually supplied by a certain breeder/or group of breeders, thus the bettas in that area should carry similar genetic codes and therefor show similar behavior. Perhaps this is why hilevij has experienced no aggression in his tanks.
#13 that's the beauty of it, most of my bettas are from entirely different pet stores, and those bettas are mainly imported from asia. And I bought them from very different times, most are probably from different imports since the bettas around here most oftenly won't stay in the store for a very long time. When I read about how the pet stores in america have houndreds of bettas stapled in small cups I realized that it is very different from how the bettas are kept in the stores around here in atleast värmland. They are most oftenly kept in 20-50gallon tanks with other sorts of fishes. Most stores only have around 2-5 red, blue and turqoise VTmales and one or two females, while other stores hardly have any females or a tank full of females and five or more very colourful males. What I tried to say (I babbled my self away from the subject a bit :]) is that it is most unlikely that they are related in anyway. Wath is it that people say, that heritage and enviroment is the main causes to why individuals evelope in the way they do.
#14 I got one beautiful pineapple ct boy who's never done anything to the other males, when I first let him into the group (he had been in quarantine or what ever it is called, for a week or two) he ignored the red vt male and the blue plakat male, and none of them cared about him, which is unusual since the others atleast have fleared at each other when they were introduced. Since he's been a daddy a couple of time, he usually gives the ladies a little dance when I reunite him in the group, but he knows his place and don't interfare with the males. The plakat male is a wild boy, and tasted some of the red vt males pretty long fins when I let him into the group, but he would accept the other male very very soon, and since then it haven't been a problem. And the vt healed in a couple of weeks, he were never seariosly injured. The fourth boy was bought as a girl (he's pretty young and have those stripes that bettas get when they are stressed or wants to show other bettas that they aren't aggressive) and he is a turqoise vt. He has also tasted my doubletailed betta girls fin when they were transported home from the store. But it wasn't anything big, and I would like to call it stress related behaviour, and her fin grew out in notime. They were so cute when I let them into the big tank, cause they sticked together for a couple of days before they joined the group. Ofcourse I think my bettas are the most pretty and nicest in the world, and you learn a lot more about their personalities when you see how they tackle social situations :)