This looks quite nice. Would it be possible to mix, say, wood pigeon with tuatara? Or another native bird which rarely spends time on the ground? A bird species in here would be quite nice.
I would have though that larger birds that rarely use the ground (tui, wood pigeon, maybe even kaka - or would they be a bit too 'inquisitive'?) would be OK with them. Otherwise it seems like a bit of a wasted space.
I was just flicking through the manual as you were posting, and I can't see anything about not mixing tuatara with other species except where it says that adult tuatara must not be housed with juvenile tuatara or any smaller display animal.
I definitely wouldn't put kaka with tuatara, and I wouldn't trust a tui either. NZ pigeon might be fine. But really its just an extra risk (both physical and in terms of communicable disease, eg, through faeces) that can be easily avoided by not mixing them.