Threadjacking? If it's a public network post, feel free! If it's a locked network post/thread or a log, I'd prefer it if you ask first.
Backtagging? I thrive off backtags. Yes, always, and as old as you want.
Fourthwalling? Preferably not, but it depends on the context. Ask if you're really set on the idea.
Offensive subjects? If I don't wanna write something I'll just say so aka I'll let you know if I'm uncomfortable with something in a thread if it occurs, but overall I'm pretty difficult to offend.