Hello, stacwase.

GSD need excerize, and any dog can dig.
When I was growing up we had 3 GSD's.
and one that was quiet an escape artist.
You need to make sure your dog run and front porch
is escape proof for any dog.
GSD are territorial and can be very protective.
Some GSD are dog domiant, but if you
choose carefully there is no reason why
your GSD can not get along nicely with your sheltie.
Chose a breeder very carefully or go through
a reputable rescue that knows their dogs and
will match the perfect rescue for your household.

We have a few GSD owners on this list, I am
sure they will have great advice for you.
Training is a must too, GSD are usually very
easy to train and their quite smart.