A lot of longarm quilters use cotton/poly thread. In fact a lot of the pretty ones are cotton/poly. That is what serger thread is, check the label. I don't like to use cotton/poly thread, but that is just my own preference. I don't usually use the clear nylon type threads in quilting either, but many use it and are happy with it.
So, bottom line is 1) if you like it go for it 2) No quilt police here. :lol: