@Raymonde. Some inter-generic crosses can produce viable young. Most young thus produced are mules, but in a few cases they are actually fertile. For example, the Red Siskin Finch Spinus cucullata has been bred with canaries, Serinus canaria, to introduce a red colour factor into this otherwise yellow bird – not surprisingly called “Red Factor Canaries”. I guess it gets down to the fact that we humans draw the lines around species and genera... and nature doesn’t always fully agree.
@KingsReptiles. A few ideas to consider...
Definitely agree that it is important to match sizes of occupants. And it is not just the dragons that are potential predators. There is a thread somewhere here about keeping a bluey in the bottom of a landscaped finch aviary. When the owner started to lose fiches he suspected rodents but could find no evidence of them. Irrespective he took the appropriate steps but it made no difference until he caught the culprit in the act – the bluetongue. A couple of other members said they had similar experiences.
The only like cross breeding in a pit from all the animals you listed would be the Eastern and Blotched Bluetongues, so better to keep them separate or choose one over the other.
If you consider the ARP mixed pit, in rough terms, adult SV lengths for the skinks was 300mm, for the larger dragons 250mm and for the smallest component, the Jacky 150mm. So adult jackys are just big enough to avoid being on the menu. In terms of young, any dragon offspring would be a potential meal but skink young are large enough to avoid that. So you would not be advised to copy ARP if you wanted to breed the dragons.
My opinion... Adult Eastern Water Skinks have a SV 120mm but are quite stout bodied. Full grown, they are also likely to be too large to be preyed upon. So worth trying with the ARP mix (with one species of bluey, that is). The similar SV length but more slender body of the Robust Ctenotus will likely not afford the same degree of protection. Clearly the young of both would be vulnerable.
I doubt the ARP mix included Shinglebacks. Their greatest enemy on the east coast is humidity and an open body of water would certainly make for a more humid environment.
I have only kept one juvenile short-necked turtle, which was voracious feeder on tadpoles and Plague Minnows (Gambusia). My limited experience with C. longicollis is that they tolerated small water dragons in the same water but was also used to being regularly hand fed. So follow the advice of those more experienced here.
Shingleback are more herbivorous and insectivorous that other Tiliqua. I suspect they could be housed with small dragons and skinks without concerns of predation. But I emphasise that I have not seen it done so do not know for sure. So I figure you could have a reasonably dry style pit with Ctenotus, Mountain Dragons, Jackys (if desired) and Shinglebacks.
As pointed out, blueys are not good in water. So if a section of the pond has steep sides or overhanging rocks, what can be done is to divide this off so that the blueys cannot access that portion. A simple low rock, brick or plastic lawn/garden divider wall is all that is needed. Brick can be rendered with a suitable coloured oxide in the cement to make it blend in. Planting tussock grasses like Themeda spp (kangaroo grass), Danthonia spp (wallaby grass), Lomandra spp, native Pennisetum spp (foxtail or fountain grass) etc can be used to screen this low wall without providing any physical support for the lizards to scale it.
