Yes, you can have Apisto harems, just choose the species wisely - some are better as pairs, others in harems. cacatuoides is a commonly enjoyed one that does well in harems.
In a 30gal yes but IME i have found it best to keep them as a single pair.
During breeding times, the first female to lay eggs will harass the other to death.
based on what i have read on this board, it is strongly dependent on the species. almost all of the cockatoo keepers advocate trios to keep the male "busy". almost all of the ram keepers advocate pairs to counteract the aforementioned female v. female aggression.
I currently have a harem of cockatoos (cacatuoides) and will soon be getting a pair to add to that (making 2 trios). If you plant the tank well enough to block the line of sight, you will have no problems with a breeding female.
Use this website to help identify your new female. Can you post a pic of the new one here? Sometimes one of my females is marked a little differently from the others. The two apistos you have are Apistogramma Cacatuoides (Dwarf Cockatoo). If the new female is a different type of apisto, she and the male can, and will, breed. I would suggest that you don't sell or get rid of any fry if that happens, to maintain the strain of each fish.
The new female is a cacatuiode as well, hooray. Can't get a pic at the moment but she has orange fins whereas my first female has yellow. There is minor aggression but the male seems to tell his women off in turn for arguing