Can heavy poundage fishing spider type line be used in a 50cc plane for pull pull rudder

Yes, I do it all the time. I've used nylon coated stainless steel fishing leader (my favorite), plain old nylon leader, spiderwire etc. all successfully. Most times I use something over 15 pound test but you could most likely get by with less. As long as it is stronger than the servo can pull, it should be fine.

I like the nylon coated leader wire. You can generally buy it in a kit with crimpers and everything else you need for the pull pull. I just use threaded couplers on one end and drill a hole through them and then use the leader wire set.