a 3rd dog possible to add

Question
Hi, I am the proud owner of a 1yr old male JRT and a 9month old female Rottweiler, since they are so close in age they have been raised together and are the best of friends/ playmates etc.....at some point down the road, i would like to bring in a 3rd doggy, but my question is as to what type of breed would be a good offset? what sex should be introduced to a pack of 1 male, 1 female (both fixed) of course.......i do live in an apartment, but have not had many problems with space since I exercise them 2 hrs a day every day....but obviously another dog takes up physical space nonetheless, but i would also not be too crazy about a toy breed or poodle....medium size would be a good fit, also aesthetically.....also a low barking breed preferably..
one terrier is enough, so preferably a calmer breed who would compliment the Rottie and maybe even calm the Jack! I don't want to show preference so I wouldn't get another of what I already have....it would definitely make the oddball feel more odd.
my guesses would be (but just guessing here)
a black lab
boxer
doberman
some sort of mix? (medium size) any other suggestions?

Answer
Well I think a really good idea would be to go to your shelter,and talk to the staff and explain to them what you are looking for,they usually know the dogs really well and can give you a good match up-the shelter is the best place to go if you are unsure of what breed you are looking for

Labs,boxers,Dobermans(as you mentioned),American staffordshire terriers,golden retrievers and shiba inus would also be good dogs to look into as well,but a shelter is a good place to at least start and go from there