I used Melafix to treat my male betta when he'd had his fins ripped off by a female - I used half doses because I already read that Melafix can be a little too strong for some fish - I think bettas may be a little sensitive to it. Anyway, my betta made a rapid recovery and eventually his fins grew back and he lived a further 3 years.
So, I can't understand why some would say it's poisonous to bettas (maybe if using a full dose or even accidental overdosing when it's not measured out correctly??).
To be honest, if your betta has bad fin rot I don't think Melafix is going to be much of a cure. It can help aid in the healing process of fin damage (along with water changes) but for major diseases I would use something else e.g. Myxazin, or similar good bactericide med.
Regards, Athena