Yes you can use wonderoos on a young infant!
Officially they fit from 8 lbs. I have been using cloth diapers since my son was about 8 weeks old (10 or 11 lbs I think) and I started with wonderoos with wonderfulls hemp inserts. They worked great! They tell you that you can use the small part only of the wonderfulls insert in the beginning, but that didn't work for us, I had to use the larger part. This makes the wonderoos somewhat bulky, but after all babies don't move that much at this age so this is not really a problem.
I tried different systems, the Wonderoos were and still are the most reliable. (Fuzzi bunz are just as good, but I didn't want to buy several sizes.) I use them mostly for nighttime and when we leave the house, and a cheaper snap-in-AIO-system when we are at home.
hope this is helpful.