Normally, you want the new goldfish to be at least 3/4 the size of the fish that are already in the existing tank. Maybe you can try raising the two smaller oranda in a separate tank until they get a little bigger.

Goldfish normally will only eat things that comfortably fit in their mouth , such as fish eggs and fry. It also depends on each individual goldfish and its temperament. Some are more docile than others.

If you want to try putting the two smaller goldfish in with your bigger one, first change the decor in the tank  around a bit so that your older oranda will think she's in a new tank. This will give them a more even playing field, since they all will be trying to get accustomed to the new tank layout instead of your bigger goldfish zeroing in on the two newbies. Also make sure to leave one side of the  tank with lots of open space for the bigger oranda to swim so she doesn't harass the smaller ones. If you can add some more plants on one side of the tank to give the two smaller fish some additional places to hide, that will help also. When you are feeding them, make sure the smaller ones get enough to eat in accordance to their size.