A 55-litre tank is certainly better than the 22-litre. You'll find that the neons' colours will brighten once they're in a better environment, more suited to their requirements.
If you kept the 7 neon tetras and increased your panda cories to 6 or 7 in number, you should be about fully stocked. NB. Cories do better in a sand substrate as gravel substrates wear down their barbels.
Alternatively, if you sold / gave away the single panda cory (eg through Aquarist Classifieds), then you could probably get approx 6 endlers - they'd be perfect in your hard water. If you sold / gave away the 7 neon tetras too (which do better in very soft / soft water), then you could probably increase the number of endlers to about 12, provided you had no other fish in the tank.
The best way to work out appropriate numbers is to have a smaller number to begin with (say 6), see how the water quality does (if you can keep ammonia and nitrITES at 0, and nitrATES under 20), and if you're having no trouble keeping them under those levels with your bi-weekly water changes, then you might be able to increase numbers little by little up to a max of 12 (or whatever the Community Creator recommends with your tank and filter specifications). However, it sounds from what you've said, that your nitrates are quite high.
I tend to go to 80% of what the Community Creator (available via
http://www.thinkfish.co.uk/fish-profiles) recommends - it allows for the tank not being full to the brim with water and also for the decor inside. I probably could stock beyond that, and did for a while in the past, but I'd rather allow a margin for error in the event of unforeseen circumstances such as one of the two filters failing or me/someone falling ill and me not being able to do a water change.
Hope that helps.