Generally, the best characins (tetra, etc) for angelfish are the disk-shaped species, such as the Rosy Tetra, Robert's Tetra, and some others. In this group there are also some fin nippers, like the Serpae Tetra, so avoid those. Bleeding Heart Tetra is another from this clade, but it comes with a caution; several have had this fish nip angelfish, and since your angels are older and the newly-acquired BHT would be young and thus more likely to "play" at nipping, I wouldn't risk it.
The larger hatchetfish in the genera Thoracocharax and Gasteropelecus can work, though all being basic silver there is not much colour, but they will liven up the upper level. None of the pencilfish are good here, nor the linear tetra at this stage. Substrate fish like corys will be OK. Perhaps a Whiptail Catfish? Or the "red" form, Red Lizard or some such silly name, but a nice reddish-brown form of the common Whiptail.
Avoid the larger tetra like the Colombian and Buenos Aires, as both are likely to nip angelfish.
Whichever of these you select, or other species, make sure you get a largish group, no fewer than 10-12. This has two benefits, for both the angels and the tetras.
Byron.
