mbuna are pretty hardy fish and the females that are holding are usually ok with some stress. Of course the first time they hold there is no way to predict if they will go all the way. Sometimes they swallow them for no reason. Anyway, adding more fish soon will probably not have an impact on your holding female, any more than anything else. She will go to term or not regardless of what you do.
Of course adding more fish means there will be more potential predators for the new fry when the female releases them. If you want to save the fry your best bet is stripping them into a separate tank. See the articles in the library if you want to do that. :thumb: