German Shepherd is fine, it will be assessed as part of the process, as long as you've always given your dog everything it needs and it's well trained, there isn't an issue. They need a reassurance, as MyFeet said, that if the worst happened and your child and your dog couldn't live together anymore, that you rehome the dog, not send the child back into care (especially in the very early days when you're all strangers, and you may be feeling very stressed and overwhelmed anyway)
Some people have bitth children after adoptive children, but it's often unplanned in my experience, rather than they set out to do it that way
Social services, as MyFeet said, will want you to have explored the issue of birth children fully and by and large, have had yours before adopting.
Myself, if birth children are absolutely in the plan for you, then I would always recommend having them first and adopting later as well, for two main reasons:
The first is that mostly the desire to have a birth child is a very strong biological need, and emotionally, the impact of infertility can be devastating. Whereas the desire to adopt isn't like that, although of course it may be something you very much want to do and would be upset if you couldn't do it. What if you adopted, then for some reason, be it biological, financial, the needs of your adopted child, etc, you found yourself unable to have the birth child you so desperately want? Would you regret? You would grieve your loss, and how would your grief impact on your adopted children?
The second and related reason is that, it is more likely that an adopted child will have additional or special needs, emotional or behavioural issues, which may result in you deciding that it's not right for your child to have a sibling. Not all children have these issues of course, but it is more likely for sure. They are more likely to struggle with you bringing a new baby into the family (possible fears and strong emotions - Am I not good enough for them? Are they replacing me with a child who is really theirs, they must think I'm not enough of their child, they'll love this baby more than me etc etc)
You would like both adopted and birth children, which is fine, and lots of people do have both, but if push comes to shove and it's truly either or...if someone told you now that you could only ever take one route and not the other...would being able to carry your baby with your genes and parent from newborn etc...be the one you would choose? Are you prepared to give that experience up forever? Because if you are at all uncomfortable with or saddened by that idea, then IMO absolutely birth children first, adopted children later. It won't be as big a loss that way if you find yourself in a position where you can't have more than one child, which is more likely to happen if you adopt first anyway
Because the decision to have a child is not about primarily helping children in care, it's about you and your desires and needs.