Joshday.com has some very good advise for tank size.
http://joshday.com/angelfish.htm
Minimum tank: 29 gallons
Max growth of fish: 6 inches
Special requirements: Should be kept singly, in a group of five or more,
or as a pair.

With that being said, you might not want to keep them with the harlequins.
This site
http://www.myfishtank.net/freshwater-fish/rasboras/black-harlequin-rasbora/
says that you need a minimum of 10 gallons, but I would check out both sites and do a little more research since the last thing you want is a fish on fish death match.

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