I have used Rainbows on my long arm with success. I have found with the thinner threads I have to dial back on the tension. I would suggest checking out Jamie Wallen's you tube video on tension. He is talking about long arm tension, but I think it is a good lesson for domestic machines as well. Sometimes my tension looks good, but I find my bobbin or top tension needs a tweek. here is a link:
http://www.youtube.com/watch?v=Q1mRhcquZTM
PS I forgot to ask what you are using for bobbin thread? How does the thread come off? If it is a cone it needs to come off the top. I use a cone holder, but take a coffee cup with a ruler with holes clothespinned to the side. Thread the thread thru the top hole to feed the thread off the cone from the top. I take it to class..and works good in pinch! Hope this helps...