[observer]

I am not the expert on a multiple dog household, but I don't think the spayed 6 year old is going to create any problems.

However, to expand the subject a little bit, I hope you have been reading here long enough to know what you need to do before you even THINK about breeding those two youngsters. Assuming you got them from their breeders, I question the quality and ethics of those breeders, since truly ethical breeders will not sell a pup to someone that already has one pup in the house, or plans to. What titles and certifications do these pups' ancestors have?