If I pick neons I believe they need a more densely planted tank whereas cardinals need more open space?
No, most South American forest fish require basically the same aquascaping although there are species that will swim more actively than others.
Paracheirodon innesi (neon) and
P. axelrodi (cardinal) are much the same with respect to activity level, as well as dim light, and soft and acidic water. You can do a biotope-type tank, replicating the habitat closely (sand, chunks of wood, branches, dried leaves on the sand, floating plants (no lower plants). Or also authentic, more of a flooded forest habitat, much the same but here more plants that are rooted in the substrate.
These two fish do have a bit of a temperature difference. Neons are cooler water, whereas cardinals prefer it a bit warmer.
Also any advice on corys? I like the look of panda cories, is there much difference in behaviour of different cories. I was thinking maybe have some panda cories and some pygmy cories?
All species in
Corydoras have the same requirements, behaviours, etc. The pygmy cory (
Corydoras pygmaeus) does spend much more time off the substrate however, often swimming with other small fish like Ember Tetras, etc. All cories will browse surfaces including wood and plant leaves, including floating plants especially if they have well-developed root systems.
The pygmy cory is best not in with larger species, more from an aesthetic aspect. They tend to get "lost" in with larger fish. And in long tanks like this one (Jewel 240 which I believe is 120cm length) the pygmies can indeed get lost.
I'm also wondering about how the fish will occupy different areas of the tank.
This is an important aspect. Finding fish for the upper/surface level is the most challenging. And there are many species that will tend to remain in the lower half.