Hello and Welcome!!  When you introduced the pack, did you do it on natural territory away from the house?  Since you just recently adopted your corgi, it will take time for him to figure things out, but he will.  I would treat him like a puppy who hasn't been potty trained at all yet.  Keep him in an enclosed area and take him out frequently and reward and praise for a job well done.  :)  With the biting, you can discourage by making sounds like  No or Owww, and walk away.  Make sure he has things to chew on for teething.  You can also redirect to something appropriate to chew on.

How does the German Shepard feel about the mounting, does he growl and let your corgi he doesn't like it?  I would disourage your corgi from doing that.  We adopted a red and white corgi and our cardigan who is older does that as well.  Tucker will let Lance know that he doesnt like it and other times he lets him, but I don't allow that.  Your dogs will need to know that you are in charge, or they will take charge.  Corgis can have a jealous streak at times. 

I hope that I have helped you some.  You should post a discussion and you will get more responses that way.  There is also a FAQ section on the top of the mycorgi page, that you can find helpful information there as well.  I tried copying and pasting it, but am having trouble doing that.  Congrats on your new addition.  Good Luck, look forward to seeing pics and hopefully video :)