Welcome....we have five dogs right now, newf-2 year old female, irish wolfhound-19 month female, irish wolfhound- 8 month female, newf-4 month female, and a borzoi(russian wolfhound)- 8 week old female. It is crazy here when it is pee time, and the accidents that the new pup has, and the 4 month pup has all the energy along with the 8 week pup. Its crazy!!!! Training is going well, but a process you can't do alone. I have kids to help me with the jobs that need to be done with 5 dogs.
I agree on waiting for at least 6 months, then the first pup will be housebroken and then you can start all over again. we waited 4 months between two of the dogs and that worked, with us though we had to be careful that the puppy wouldn't get stepped on by the bigger and older dogs.