Black darter tetras do look great. Too bad they don't make for good community fish candidates.
Serpaes I hear can be a bit fin nippy, so I'd recommend one of the other species of tetras that look similar such as, Rosy tetra, Red phantom, Candy cane tetra, Ornate tetra, etc.
Unsure about Diamonds and Bleeding heart tetras though.
Penguin tetras do get a bit bigger than all these, getting a bit over 3 inches in length. They do have similar markings as Black neon tetras, with the extra black hook mark in the bottom portion of their caudal fin. Though they don't really have that iridescent stripes like black neons do. It is quite a sight to see when they swim tail-standing though.
Emperor tetras (Nematobrycon palmeri) are cool. If you like those also take a look at the Blue/Purple Emperor tetra (Inpaichthys kerri), they have more of a blue/purple glow to their bodies although their fins don't get quite as fancy. Take a look at Rainbow (Emperor) tetras (Nematobrycon lacortei), I personally think they have more to offer in color than the other two, but they all are great. Do research the fish though, I hear they prefer water more of the acidic side, but neutral is fine.
Whether you can stock anymore depends on your filtration if it can keep up with the added bioload. Water changes might need to occur slightly more frequently to keep nitrate levels down. Sounds like you have some empty swimming space (top), but I'm not sure what size tank/footprint.
I myself, having "been there, done that" with multiple species of small sized shoals/schools (I liked having diversity in my tanks), I have seen the difference in behavior/activity that comes with having larger schools of one species, which is better for the fish's mental and physical health, and for the keeper to enjoy more (more active, healthier, happier fish is more enjoyable to see), and so I would personally recommend increasing the Pristella tetra group (bring it to at least 12) or some other social fish group you have that may be undersized, rather than add another small group of different fish. 5 or so is the minimum, but that is the minimum just for them to do okay, not really thrive happily. "The more the merrier" indeed applies.