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.
Squicks? If I don't wanna write something I'll just say so aka nothing is super squicky to me but I'll let you know if I'm uncomfortable with something in a thread as it occurs.