How long is this hex tank?
Congo Tetras (Phenacogrammus interrptus) are moderate sized, semi-active fish that need at least 120cm of tank length. I have six that youngsters (~3.5-5cm SL) that make good use of a 120x30x37cm tank. They are rather adaptable as far as water hardness goes, upto gH 15 is considered no issue at all.
Zebra Danios are relatively speaking, one of the most common hyperactive fish in the hobby. Despite their ~5cm adult size, they really need a long tank, my group make good use of a 120cm tank to zoom around in all day long. Again, these are very adaptable fish as far as gH water hardness goes.
Lamprologus Caudopunctatus are from Lake Tanganyika, which just like Lake Malawi, has very hard water (which happens to have a pH of > 8, hardness and pH do not always go hand in hand).
Swordtails are another hard water species, they are semi-fiesty (which might help them cope with more mild mannered Tanganyika/Malawi cichlids), but more importantly they are quite active and moderately sized adult fish. Like my similar adult sized Ilyodon xantusi, I would not put them in less than a 120cm long tank for more than a short term quarantine.
I suspect your hex tank is woefully short in horizontal tank length for any dithers you already have and are considering buying, while it might be rather borderline for the cichlids as they mature.
I've found that fish groups look far better in 10+ groups of social species, rather than getting numerous 6+ groups, they look better and act more naturally. My biggest social group is 25 Pareutropius mandevillei, but I also have tetra/barb/loach groups of 10+ also (not including my species only Rio400 of ~90 Ilyodon xantusi, that I want to rehome).
