No one is saying it can't be done sucessfully if you get the right fish, but you're dealing with a very delicate situation which could easily change with time or any addition to the tank (and you're talking about adding a centerpiece fish, most of your options there will be seen as a threat by your bettas.) Many people on this forum keep and breed bettas and have experienced problems with setups like yours, very rarely does anyone mention an experience sucessfully keeping a male with females (for their full lifespan of several years) and that's almost always in a large tank of 55 gallons or more with no other fish. Information in books is often outdated or flat out wrong, if it's really giving you good information about keeping males and females together it should have included everything that you're hearing in this thread. 1 1/2 months is very little time to decide that any fish will get along well together for life, many fish will snap one month, two months or possibly even a year later in the same conditions.
I as well as everyone else on this thread suspects that you have been mislead by false information, nobody's blaming you though it often starts to happen when people ask for advice and ignore obvious problems because one person gave them the information they wanted to hear.