I was thinking that the phantoms would bring out the embers & then you'd have nice contrast.
Red phantoms are quite dissimilar fish, common name notwithstanding, so you could try them for their red rather than black color, but then if the embers do come out, you'll have very similar colors (I much prefer the black phantoms for behaviour & style
& the blue on the males when dancing is brilliant)
Cardinals won't move as nice in this tank dimension as they will in a 120cm & longer tank - likely in your tank size they will mostly just swim about individually ... domestic cardinals get quite large, & they are vociferous feeders.
Harlequins will likely be a nicer shoal in this tank size, purple/blue morph shows nice contrasts of dark body (the purple/blue can cover 70% or so when fish are displaying) & iridescent orange/gold head "spot", look to get more males as they are much brighter than females - in a school of 10-12 you can do 3-4 females (I had a school of 17 & only 2-3 females, the boys spend all their time displaying & wait for the girls' signals), natural morphs will again blend with the embers.
I have
M kubotai &
S axelrodi in one tank, they are always out in the open: note I fed them very frequently upon arrival as both were quite emaciated, especially the axelrodi - which I expected significant losses, to my surprise I ended up with a stable group of 12 ... 17/22 were alive upon arrival, 3 more died with 12 hours; in contrast, the kubotai #'s slowly diminish, after an initial 10 or so losses, 1-3 a month slip away, these are fish that have remained tiny so perhaps they are
M nanus, group is now ~20.
These fish don't really school though they just drift about, with a shoal forming, then separating, then reforming etc but it's a subtle dance. After a recent water change, 5-6 kubotai were gently arguing over a "sweet spot", then an axelrodi came in & decided he'd like that territory & spent a couple hours gently driving off any competition ...
Line bred Endlers might be interesting, I've a group of "orange guppies" that are likely an Endler/Guppy cross, they are actually brilliant schoolers (I've 7 from an initial 9) - they were tiny orange fin, orange & blue body "guppy" juveniles when bought.
C choprae appear quite bold in the shop but I've not kept them.
Perhaps list the locally available fish