Hello,My name is ***** ***** I have been a Vet tech for 30+ years and a fish keeper over 40+ years.
Let me address the Gold fish first. Tumors like this can be cancerous. Unless addressed bu an Aquatic Vet it will continue to grow a can reproduce with more cells spreading through the body. I am going to give you a list of Aquatic Vets that can treat gold fish. It is imperative in this situation to have him seen. These are Aquatic Vets in AU: http://www.thefishvet.com.au/
I wish I had better news on that situation, but it is best to have the fish evaluated hands on.
As Far as the Plecostomus, this sounds like he has some stress marks or possible bruises from all the moving and trauma he experienced. If the spots were white we would be looking at fungal issue. I would double check water parameters to be sure the Ammonia is at 0, Nitrites at 0, Nitrates at 20 ppm and the ph at 7.0. If the water parameters in the tank are off that can lead to more stress. At this point I would monitor him as long as he is eating and swimming normally.