Determine your desired water depth then get a piece of glass cut and attach it with silicone to make a barrier between the water area and the land area.
you would be best to go with either one larger animal or multiple small ones... think anoles... green tree frogs.... molly's and platy's... small things.
Mixed vivariums do not work well with larger animals unless they are HUGE.
Try to get things that will not be in each others way... like if you have tree frogs... don't get crested geckos as well..both are nocturnal and occupying the same area of the viv. try to go with say, a diurnal terrestrial species and a nocturnal arboreal species.
Determine your desired water depth then get a piece of glass cut and attach it with silicone to make a barrier between the water area and the land area.
I figured this is what I would do. But are there any other ways?
Predatory fishes eat quite a few things. It has been mentioned on numerous occasions, and on numerous worthy articles that newts and salamanders require cold temperatures. Do you really want to cool your stingray or arowana down into 20 degree celsius water!?
Furthermore, the footprint is one thing, what are you going to do by putting a stingray in a water volume in which it may barely move vertically, much less an arowana.
The options you are putting forth are ridiculous. Patriot crab/pacman frog is a matter of who gets who first. To put mix two species of reptiles or two species of amphibians already gets pretty crazy. Do your research.
In reference to you wanting a ray. I've had my 10" disc male motoro living with a 26" amphiuma salamander, granted these are fully aquatic salamanders and my particular salamander does not like the taste of fish, won't even eat chunks of fish cut into small pieces. I have however heard of other amphiuma going after fish and taking chunks out of them so it really depends on individual personality for if something like that could work.