If I use knitting yarns in my weaving It is usually in the weft. I often float them on the surface so they don't end up squished. Some "railroad" yarns look good being squished because they are usually varigated, and, used randomly, not consistently, add just a flash of color. I find that almost any yarn can be used as weft, but a good warp yarn must not stretch very much.
A mixed warp can yield differential stretch, and that may lead to tension problems. For example, I purchased a space dyed, mixed fiber warp chain, and threaded my loom front-to-back. The first threads through the heddles that got tied to the back beam had the colors lined up just the way they looked when I purchased the warp chain. By the time the warp was fully wound and tied on to the front beam the colors looked muddy, due to varying amounts of stretch in the different yarns. I found that I had to use a weft with "tooth" to stay in control, because the stretching continued throughout the weaving process. This problem was very visible because it was space dyed. So, watch the stretch of warp yarns, especially if you use "knitting" yarns.
SallyG