While you are correct regarding them being compatible with each other depending on the individual personality of the Betta itself I disagree with you in many ways as even a relatively peaceful betta such as the one that was with the guppies can still become aggressive towards them as there are no facts, ways to test, or precautions to take to ensure that every individual Betta will or will not be compatible with the given species. in this case I had what I though was a peaceful Betta that interacted and lived happily with Gourami's. So I thought that he would be okay in my guppy tank, yet he still attacked them and to what I thought wouldn't happen happened. I would never recommend this case as you can already see, a peaceful Betta can become an aggressive one in different given situations so experimenting can result in death, disease, or stress. I wouldn't recommend this even if you have what seems as if to be a peaceful Betta, Because almost anything can turn and go wrong.