As far as breeding back to back, it is somewhat a matter of opinion. While it is 100% true that making your baby into a puppy factory (breeding back to back several times in a row) is not healthy for her, breeding 2 times in a row then skipping 1 is seen by some as a healthy breeding method. Another method that is often used (and which my aunt has used for years) is the skip a heat method. This way you have a litter, skip the next heat, then breed on the 3rd heat again. It is completely up to you what method you would like to use (as I personally believe both are pretty safe). The best thing to do would be to consult with your vet about the health of your girl, and if she may be ready to have another litter. Your vet may believe in a specific time frame for breeding, and he knows your dog best, so unless he/she says 'breed her till the cows come home' you should probably listen to him/her.
I personally believe in skipping a heat between litters, definitely if the mother is on a 6 month cycle. My family lets little Maria have puppies once a year (every second time) though she is about ready to hang up her puppy rearing days and retire to a life of couch sitting and tv watching
Good luck with your dogs!
~LeAnn