I'm keeping a close eye on the remaining threadfins. There are two adult males who seem to be doing fine and five juvenile males who are looking less confident. If I see any further signs that they can't cope with the flow in my tank then I will regretfully return them all.
I was going to buy more females to ease the pressure on the original lone female but now that she's gone I may just keep it males only.
The harlequins have been extremely active, racing from one side of the tank to the other and schooling tightly. They seem to be gradually calming down and acting less manic but still need time to settle in.