That is a very very tricky ordeal which may or may not work. First I would suggest a larger fish such as congo tetra's or perhaps larger rainbowfish. Then when you have the new fish you will need to take the angels from the tank for an hour. Put them in some buckets or a large Tupperware tub in their tank water. While acclimating the new fish refill the tank. Then once the new ones are acclimated and in the tank the angels can be reintroduced. BUT this could upset the pecking order of the angels as it will be like putting them in a new tank where nobody has established dominance or territory. If you can't remove the angels, then you need to feed the angels their most favorite food at one end of the tank while adding the new additions to the other end of the tank. Then turn off the lights until the next day. Either way you will have to watch everyone for a week or so. It may or may not work. Just don't try adding any smaller fish with angels that are that old and never been around smaller fish.