I've been using the one main adapter with 3 other adapters / extension points in my house with no problem (4 total adapters). You're supposed to used the powerline adapters together on the same power circuit however, I'm using one downstairs and 3 upstairs all on different circuits and haven't had any issues. Your question about limitations is correct though.
As for the security question, to my knowledge, all of my security features, which are set through my modem as well as my internet security program still function just fine. I would always have security though. You can never be too safe as to what websites will try to do with your PC and what your neighbors might try to do to use your network.