I use the Carefresh cardboardy type stuff and have never had any problems.
If you have only had the rats a week then is it possible that they are just working overtime on picking up new scents? Personally, I would give it a little longer before I started making changes, let them get settled in properly.
In the meantime, you need to keep an eye out for any discharges from their eyes, nose and private parts. Their tears and fluids are naturally a pinky-red colour, but if you start seeing too much of it, it may be a sign that they have a bit of a respiratory problem. Another thing to watch is their poop. If it looks a bit off colour or sloppy, that can be a sign that they are a little unwell.
Overall, seeing as you have only had them a week, I wouldn't start worrying just yet. Just watch them carefully, make sure they are eating and drinking and take it from there.