IMO, you need a 20 long if you want tetras + ram. Maybe a 15 long would work with just a pair of rams (german blue/gold/electric blue, not Bolivian - they need a 3' tank minimum).
Probably 5-7 neon tetras max...are you trying for a pair of rams or a single? A pair might be risky. A single male would be quite colorful and probably ok.
Black neons work well with rams and angels in a 3' or larger tank.
As far ammonia, add some extra bacteria from your main tank or try the bacteria in a bottle and you shouldn't have to worry about a spike. Tetras (especially small bodied ones like neons) don't add much to the bioload compared to adding, say, 3-4 plecos.