I love Congo Tetras, but they could look a bit crowded in your tank, and maybe out of proprtion with your other fish.
Have you seen photos of mature Black Widows? The black fades and they look quite dull. They can also get a bit aggressive as they get older.
I would suggest Black Phantom Tetras. They are a nice size for your tank and tankmates, keep their (mostly black) coloration, and have a bit of shining green to contrast with the black. Females have a bit of red in the fins too.
Of course, it comes down to your preference in the end, but have a think about the Black Phantom Tetra.
Angelfish would suit this tank very well (as I recall tank is fairly tall - 55cm check )
To minimize chance of angelfish eating small tetras, rasboras etc, just have your shoaling fish established before adding small angels (1.5 cm body)
Adding a group of several angels helps maintain them as a group rather than pairs - bonds may form but they’re less likely to actually begin breeding (especially if food is sufficient but not plentiful)
Of course nothing is guaranteed so if you never want a breeding pair of angels, only keep 1
But then you miss out on the social behaviour of these fish - closer to wild, the more behaviours typically displayed BUT then also the greater chance of “hunting” being one of the displayed behaviours
I’ve kept wild caught altums with small tetras/rasboras - the actual fish they grew up with were not seen as possible prey, but when I added some more harlequin rasboras (similar size to existing shoal), after one week the new rasbora were MIA
When I kept domestic angels with small tetras etc, I didn’t observe hunting of new individuals (tetras etc) but I always maintained any smaller fish in a good sized shoal .... and rarely added new angels - as with discus, sharing normal flora between newly introduced angels/discus often ends up with sick fish (so suitable quarantine and introduction procedures are important)
Adding an adult angel to a tank with small fish, it’s likely your small fish will disappear over time
High bodied tetras etc are less likely to be eaten - eg, phantom tetras vs neons
For the platy breeding issue, I’d separate out and keep only males or females rather than depending on other fish to eat the fry - platy/guppy fry are pretty good at laying low, and with the constant production, some will survive
Bettas are good at hunting down hiding fry, they’ll go anywhere up or down that the fry may go BUT if the platy shoal holds the area where most fry are hiding out, then you’ll need more aggressive fish ... which then means they are more aggressive in general (poor neons I suspect ... not that they will get eaten, but they will be kept in check - a larger shoal would help with this behaviour eg, 26 neons rather than 16)
If you’re not keen on the platy’s, I’d rehome them
- and send a note off to the supplying shop that they should’ve warned about the fry explosion and offered to select only males - this is no less important than the angelfish concern
Add more neons
Decide what other fish you’ld like
But to answer your actual question, I prefer Congo tetras over Black Widow (though I suspect if you found a wild caught source they’d be more attractive and less aggressive fish) - though I’d usually not suggest Congo tetras in any tank under 120cm length