I've had my rainbow shark for almost 4 years, and he's now about 6 inches. He has been in a lightly stocked 29 gallon tank since I got him.
I agree, the painted angels may very well have had one fin in the grave to begin with, if they are injected with dye. Also I am pretty sure that angels and bettas don't get along - the bettas tend to hate most fish wtih flowy fins such as their own. Did you see the angels and betta fighting at all? That could account for both of the deaths.
~Tara