I imagine they best time is when they are partly or fully grown but still not reached breeding age. Howletts also set up a young trio like that from Port Lympne's and it seems to be fine. I think there is always some jousting/scrapping- that's rhino socialisation. Even the Sumatran pair at Port Lympne use to do this.
Yes but with a smaller number of rhinos there is less chance on injury. At chester it was always the two girls ganging up against sammy but because there were only two and the layout of house meant he was quite able to stop them.