Hahaha! Spinal Tap Stonehenge moment! Don't worry - I've done that myself, most recently, when I was explaining to a friend how the patio at the end of my garden was 6 feet by four and housed a greenhouse and shed. He very politely suggested it had to be a very small greenhouse!! (I meant metres, of course!)
Now, this 30 x 60 cm space - is this in front of the trellis? Is it where you're thinking of putting the acanthus, foxtail lily and trachylospermum? Because the acanthus alone gets to about 1.5 metres wide! So it will be far too big!
I think I might be tempted to go for something featurey and very simple- a non-invasive, clump-forming bamboo, or a bit of topiary, or even a statuesque acer (one that gets pruned carefully into a shape, not one of the shaggy ones that droops all over). Those will all get their roots under the adjacent paving.
I like the black phormium, but I wonder if it'll look a bit lonely and odd all by itself - it's hard to judge without seeing a picture, but it sounds as though this is more of a 'feature' kind of space??