Yes, the legal limit for ammonia is 0.5ppm (the same in mg/l). If your tap water level is above that, I would query it with the water company. Although some companies do use chloramine to disinfect the water and that will show a positive result for ammonia.
As for fish, You have cut a big hole in the possibilities with your don't want list
![Grin ;D](https://forums.thinkfish.co.uk/Smileys/emotions/grin.gif)
Just a few suggestions that are a bit different, but you need to bear in mind that most bigger fish will need a bigger tank:
If you have soft water, dwarf cichlids such as apistogrammas or bolivian rams. Bolivian rams and cockatoo apistos can take slightly hard water.
More softish water fish, but they do need a tight fitting lid -
hatchetfishGolden (aka beckfords) pencilfish are the hardiest pencilfish and easiest to find in shops. Will live in soft to hard, acid to alkaline water. Grow to almost 2 inches. Shoaling fish. Not the most interesting looking fish but they have a different swimming pattern involving hovering in one place.
Harlequin raboras, again a shoaling fish that will cope with a range of pH and hardness
five band barbs though they do better in soft water. Similar to but much less aggressive than tiger barbs.
And of course corydoras for the bottom of the tank. With just 120 litres, something like panda cories would be ideal.