The War Games of Zelos - Novel 1975
This is Book Three of Four in the series called 'The Expendables'. He wrote this series using the pseudonym 'Richard Avery'.
They were THE EXPENDABLES - some genuine volunteers, others not. They were the chosen ones - the people who had been 'selected' to travel far into space and find new planets for human colonisation.
The Expendables go to Zelos - it seemed an ideal planet for colonisation. For the first time they found human beings already in residence. But this was a society remote from anything they had ever before expeienced.