Help Please, Fishies Not Looking Very Happy And Lost 2 Rummies Today

The filter bacteria do grow on all other surfaces in the tank, they just grow more in filter. There would have been some on the surface of the old tank itself. Not many, but enough to take things carefully after the change. The bacteria on the decor have gone into the new tank, so you didn't lose those; but since the substrate would have been churned up, the bacteria in the surface layers may well be underneath now, so some out of action there too. In total, you will have lost some bacteria during the tank change, but only a small proportion of the total; the numbers would have caught up quickly.
You say you put the old media into the new filter - good. But the old filter casing would have had some bacteria on it as well. Again, the numbers would have been made up quickly.

But the trace of ammonia you detected could well be a result of the tank and filter changes. It's called a mini-cycle and is usually over very quickly. For the next few days, feed the fish less than you usually do. Less food in means less ammonia made to stress the filter. Then increase slowly over a few days to the usual amount. And monitor your ammonia and nitrite levels a few times a day till you are sure the spike is over. I would also do a few extra water changes too.

Rummy noses are among the more sensitive tetra species. They will be the first fish to succumb to anything iffy in the water. Keep a special eye on them and do a water change if they look as though they are suffering.