Nitrates 40ppm, No Ammonia Or Nitrites

It could be that because you have only 6 small fish in a 125 litre tank, the ammonia and/or nitrite are just too dilute for the testers to pick up. The decorations would have had some bacteria on them. If you upgrade this tank to an even bigger one, put the filter media into the new tank's filter, that's a better way to move all the bacteria over.
The nitrate tests we have are only ballpark. It is next to impossible to make a really accurate one for use in the home - it needs extremely expensive lab equipment for that. So it is possible that your nitrate readings have not doubled as the results suggest. Have you been giving bottle 2 and the test tube a really good shake?

For now, I'd keep checking the ammonia and nitrite levels for another week or two. If you still get zeros for that time, you will be safe to get more fish, but not many at once. No more than 2 guppies-worth of fish. The check the levels again every day and if they still stay at zero, a week later you can get a couple more fish, and so on.
But if you do see any reading for ammonia and/or nitrite other than zero, report back. It may not be necessary to do water changes. Let me know the readings and also your tank's pH and we can see if you do need water changes.