There is this rule in some forum I've been on that says that old topics should not be resurfaced. Is it practiced in EVERY forum? If so, what's the definition of "old topics"?

I personally think topics over a month is old.

You're welcome to continue in any thread you like. Keep in mind that some may be old and their content will be dated according to the post date, but by all means. I'd prefer it that way too, no use making a mess of things when a thread with the appropriate topic already exists. Now, if you plan on going into a different enough topic, do start a new thread on it.