I think a guppy on it's own would not last very long. I have only ever kept guppies in groups of at least 4. They always swam about together.
When we were going to move I didn't replace any fish that died of age. I had had my tank up for a couple of years and over the time had many guppies and some tetras. When I was down to my last guppy he looked depressed. They also look better in groups.

I don't know about a betta with other fish.. not really sure what they can live with.. Not guppies anyway.

x

__________________