First, I have moved the posts into this new thread, so that is handled.
Second, deanb, welcome to Tropical Fish Keeping forum. Glad you joined us.
Third, to the nitrates. This indicates the tank is probably cycled. Youo can read more about cycling and bacteria in two articles, linked here:
As for adding new fish, you can move the 6 tetra into the 20g (70 litre) tank. It would be best to move 3 one day, then wait to move the remaining 3 a couple days later. If you have live plants this will not matter as plants will handle the extra ammonia quickly, but without plants the bacteria need time to catch up.