Hi Pyewacket2,
As you said, the Waterhousia can grow to be much taller than your preferred 6 metres. The only way to restrict its growth effectively is an annual prune which can be a massive job, not to mention hazardous. The Waterhousia also has a very different habit to the Elaeocarpus eumondii, being wide and weepy compared with the narrower, upright habit of the Elaeocarpus. If you want my advice, always select a plant that fits the size limit rather than limiting the plant. Why fight nature?
I'd look at maybe downsizing the Elaeoocarpus to save on cost. Smaller plants will establish faster and often catch up to the super advanced stock within a couple of years. The Eleaocarpus will be well worth the wait. It's a much more attractive tree all round in my opinion.
Small to medium trees you may find easier to source are Magnolia 'Little Gem', Elaeocarpus reticulatus (Blueberry Ash) or Cupaniopsis anarchardioides (Tuckeroo). Blueberry ash doesn't perform well when planted in lawns and Tuckeroos can vary a bit in size and growth habit.
Sorry, I can't comment on root invasion. All trees pose a risk and can make a liar of you. The best you can do is avoid planting too close to services.
I hope this helps.
Mark