There are just a couple of criteria:
1) Not too small, because small tetras will get eaten. Silver tip and black neons (tetras about that size) should be ok.
2) Not too nippy, because they will nip the angelfish's fins. Buenos aires tetras, serpae tetras and black skirt tetras are all known to nip. Your mileage may vary, but I would be prepared with a plan B if you are planning to introduce known nippers. Maybe your LFS would be willing to take them back?