Anabolic steroids can cause the development of acne. However, the extent to which it is experienced can be due to a number of varying factors, with the particular steroids and exact dosages used being primary. The skin´s sebaceous glands have a particularly high affinity to Dihydrotestosterone, which is an androgen the body naturally produces from testosterone via the enzyme 5-alpha Reductase. Increased sebaceous gland activity promotes oily skin which can combine with bacteria and dead skin (normal wear and tear) eventually causing pores to become clogged more quickly than the body can cleanse them. This of course, is preventable by using only particular steroids, cleansing the skin regularly, and perhaps using a topical anti-androgen.
No, this is not necessary. Anabolic/androgenic steroids all work primarily by attaching to and activating the same receptor. As such, you do not gain anything by switching to a new compound that works via stimulating the same receptor. If tolerance were induced by one AAS compound, it would be extended to all compounds. The plateau effect that is noticed 6-8 weeks into most cycles is poorly understood, but likely related to the new metabolic limits placed on muscle cells under the influence of a certain AAS dosages, not insensitivity to AAS. Classic downregulation does not occur with these drugs, and even if it did, rotating steroids would not prevent it.
According to this anonymous professional, no health problems exist in any of the Mr. Olympia (largest annual and most competitive bodybuilding competition in the world) today. Just recently, Mike Matarazzo sustained a serious shoulder injury and Fukes (both well-known professional bodybuilders) blew out both of his knees but neither were from the use of steroids. I know of no ex-pro bodybuilders that are having health problems as a result of using steroids. The media (and government) will tell you all kinds of horror stories, but conveniently leave out the fact that the people having health problems were also mind-altering drug users.