I started with 10 zebra danios and 10 glowlight tetras in my 400l tank. They went in on day 3 in the life of my tank so were potentially subject to high ammonia and nitrite levels. Of course with the size of the tank and the relatively low stocking level my parameters never got even slightly high (I actually never got any reading for ammonia!). I added Stress Zyme to the tank to try to kick start the cycling which could account for the lack of ammonia.
Do yourself a favour and get a water testing kit. You can get the whole set of tests for about £20 and it'll save you plenty of time and hassle trudging down to the lfs to get your water tested.
I may be wrong but I recall the water up your way being rather on the hard side which wouldn't be too good for the common tetras which prefer softer water.
Basically I'd advise you to test your water and know what's in your tank and then read up about the fish that are suitable for your water. Choose a few hardy fish to start and go from there.
As far as top, middle or bottom feeders goes, it shouldn't matter. Just make sure you don't overfeed them. Most fish are always hungry and will eat even if they don't need it. They will then excrete almost pure mashed up food which will rot in your tank and cause pollution problems.
If you want lively fish I'd suggest zebra danios. They are non-stop fish!
WK