Wouldn't want to disagree, but one website that has them for sale suggests 3 feet apart. They are effectively multi-headed mini cauliflowers so 18" would probably be a bit cramped.
I wouldn't disagree either I saw the same advice but then again there is a lot of ambiguous information out there on the WWW
I recall when I started growing brassicas around forty years ago the advice was plant out 2-3 ft apart, then you would see further advice later in the season to stake your brassicas, in particular tall growing crops such as sprouts.
Then a few years later when people started using narrow or raised beds the pundits would say to plant your brassicas " a bit" closer but I can't recall anyone defining the term " a bit" There reasoning was because you were using narrow beds you could access your plants with out standing on the beds.
I experimented over the year ( no WWW then) to find out what a " bit" was and it was then I came to the conclusion of 15" to 18" apart.
What I found that taller plants supported each other so no need for stakes, although sometimes the plants on the edge of the bed sometimes needed some support.
Then I found that it was always better to grow F1 hybrids and these usually grew as it said on the packet, so this made size selection much easier.
Something I did notice though was that very few experts advised you to plant in the sunniest spot on the plot as brassicas love the sunshine on their what is effectively their "solar panel" leaves. I found this out the hard way due to my rotation programme as I have one bed on my plot that gets less sunshine than the others. One year when my rotation programme made me use that bed my plants and subsequently cropping was very poor.
Now to Nine Star Cauli!
I see these more as a Calabrese rather than a cauli and as such behave a bit differently from a true single headed cauli.
I grow my Calabrese at 15" centres and they can grow quite tall depending upon the variety I grow.
At cropping time I find I get a good sized cauli like head then when this is removed I get smaller tight heads, as opposed to the open headed spears of the broccolis, in effect they are not much different from the heads produced by the 'Nine Star' cauli variety.
So to conclude: I am not suggesting that other people are wrong in growing at centres up to 3ft in diameter, this is entirely up to the grower, but from my experience 15"-18" works for me and the type of soil and growing methods I use.
Perhaps you could try a few plants at different centres and see how you go on.
This is how I experimented in the past, but then I didn't have the internet to call on, in those days if I wanted to alter my methods I would have to wait till the next growing season to try something new.
Even this had its drawbacks particularly if the season was quite different from the previous season in terms of sunshine & rainfall.
Even to this day at the end of the season when I am about to purchase my seeds for the next year I ask myself questions like: did the variety I grow this year work well for me? should I grow it again? did I do something wrong? then when I have answered these questions I tick the box for the seeds I want to buy!
If I want to try something new I tend to grow about 60-80% of my total needs using my tried and trusted variety, and the remainder with the new variety.
This way I get a true comparison because the old and new varieties have been grown under identical conditions unlike the past, when I waited another year to apply my changes.
Add to that I can now read how other growers up and down the country (and even abroad) are performing so I can compare notes or indeed change my whole method of growing if I thought it was needed!
So yes the WWW can be ambiguous but it can also be very useful indeed!
Sorry for the length of the article but I felt I had to put in my reasons for the answer I gave earlier on in the thread.
After forty odd years of gardening my motto is:
Due to the resilience of plants there is no wrong way to garden, and for the same reason there 'are no bad gardeners' but there are 'better gardeners' and these usually have years of experience behind them.