They can be used for just about anything that flows through a tube
Im speaking based solely on industrial experience, not fishkeeping.... but yes, they can be used sump plumbing... they are just plastic (PVC, HDPE, etc... depends on what you buy i think)
Hehe yeah htey look way better than the average piping. So will it be okay if I use it for the return and the overflow to sump? Also do htey just snap ot twist on? Or like cement/glue?
Size depends on the pump and desired velocity. I would only use this in the tank for the return. Hard pipe will do much better for your overflow and outside return pipes.