There are no "rules" beyond not being able to compete in non PA or IPF events. You have similar rules in other sports, including bodybuilding, if you count that as a sport :O
As far as moving into PA from another fed, there is no issue. You aren't going to be "shunned" or anything like that.
You can be banned from PA for a period of time (usually 2 years) for failing a drug test but can compete again after that time is passed. Some individual members may give you the stinkeye after a suspension, but there is nothing official about banning you. This is also similar to other sports.
You can get a lifetime ban for multiple positive drug tests. Hello Ed Coan.
Anything suggested beyond the above is personal bias and shit stirring. e.g being coached by a non PA person or being a member of a "bad" gym.
Marcos from PTC often pops into comps and members will stop and chat as he is well known and he also knows PA members personally (Hi Marcos).
Oh. If you are a coach who is non PA, you can't coach at a comp as you have to be a registered PA member to coach at an event. You are even subject to drug tests at a comp, not that it would ever happen. This is the same in other sports and the olympics. But you can train, and be trained, by non PA coaches.
The good news is that if you are taking anabolics, you can compete without fear of drug testing or being banned..........
if you compete in GPC.