Beyond having another set off arms where the legs are what would you change? I'd give them a longer neck. Perhaps about 30cm long and highly flexible so they can crane their heads around to look for the direction they want to push off to. I'd have the eyes larger. About the size of golf balls. Their vision is pushed to as good as is biologically possible. Eagle-good eyes basically. Maybe they could be "double jointed" like really flexible people are referred to as. They'd only be about the size of an average 8-10 year old to save on space in the ships and colonies and food.