Where can I find the general forum rules or community guidelines?
I don't know how old topics I can post replies to and when I should start a new topic for example.
We have no specific rules (except for "be nice", "don't spam us" and "no ads").
If you try to reply to an old post, the community will display a message, telling you the thread is old.
In that case, or if in doubt, just start a new thread. Maybe link to the old thread if you consider it useful.
If you reply to an old thread, you will get answers too. But old threads are old for a reason, so...