Well aren't platies colourful enough? Bettas can sometimes be tough and mean and other times very docile so it is hard to say how they would react. They won't go after shrimp and will probbaly chase the other fish some what, but then they could also be attacked by the other fish in the tank.
As for pH swings I would be wary there as neons and the other smaller tetras don't tend to be too hardy these days. They can easily adapt to a new pH but not a constantly changing one.