I haven't seen glowlights in many years, we had them early on, nice fish! Those blue tetras sound aggressive from a quick search, I've never seen them in person. I have a personal dislike or at least caution with tall bodied reddish tetras, there are many species available. Bad experience with serpae tetras may have scarred me forever. What about red phantoms or candycanes? I've never kept them (too similar to serpae) but they "might" work with your black phantoms. A few red high body tetras get pretty big & might boss your other tetras around (polite to shred their fins) Be careful! Read a lot!
Lemon tetras? Not the most colorful for me but NoodleCats has some very yellow 1s that are pretty.
Really, rather than another species I'd suggest doubling the number of both tetra species you have. Black neons can be lovely & you know you already love the black phantoms. 5 or 6 is a "minimum" of schooling fish. More of both would be more fun! Just think about it...
Also not to be too much of a downer but you might start thinking about what you'd like after your rams die off eventually. I take it they are not a pair (so no breeding replacements) & 7 years is getting up in age. Apistos? Corys? There's nothing wrong with planning & researching ahead of time.