Fisrtly, its very good of you to ask about rules etc, and yes there are a few niceties to consider. If you intend to use 'bits' from other layouts its polite to get permission from the original designer (whenever possible), then make a 'thank you' reference to them when it comes to making a release post. If you use someone's classic as a starting point then you should definately thank them in your release and possibly even put their name in 'Caption' in the configuration section so it shows on the status bar.
What you should never do, and many newbies have fallen foul of this is to change other designers layouts and claim them as your own. That is a definate NoNo!!
Lastly, at DIF you'll find that this is the place where the current DX'ers hang out and most certainly the pace to come to if you need help and assistance. It always good to see someone wanting to try their hands at DX'ing and at first you're probably going to have some questions. If you do, don't be afraid to ask as we're always happy to help