Ha, I don't think he will use the name Spectrum, but for the gag, that might be fun.
Ewing would be an excellent choice, the interaction between Genis and Noh-Varr would be hilarious, since they were both rebellious teenagers when they were younger.
And I think Genis was called Legacy in one of his first runs, the one drawn by Ed Benes, but I'd be okay with just Genis.
Phyla from the other universe don't have code name either, because I think she was also Captain Marvel in her own dimension.