Welcome back, Sunshine.
One of the theories I had for my 6 pygmy cories' demise in the summer (lots of posts on here re that) was that, once one died, they weren't happy in a smaller shoal and it stressed them out, resulting in a domino effect of falling ill and dying. Subsequently, I felt that a much larger shoal may have resulted in a much better outcome, and all the more so if the other fish were larger. My hunch is that, with the other fish (tetras, harlequins) each being double the size of the cories, I might have been better off with 12 cories for every 6 fish that were double their size (in effect meaning that the small cories collectively would be the same size in fish cms/inches as the bigger fish collectively), and the cories might have fared better.
In your case, I wonder if increasing the shoal size to a minimum of 8 would help disperse or remove the aggression or competition or whatever's going on - it might improve the general group behaviour to be more natural at least, and it might also result in some similar-sized companions for your 'runt' or even ultimately for your larger ones. If you happened to have more space, then you might even want to consider increasing the shoal size to more than that, depending on your other fish in that tank, their sizes (possibly along the lines of my untested hypothesis above) and area of the tank which they tend to inhabit (eg you wouldn't want the bottom over-crowded).
Just another option to add into the mix for you. Out of interest, what other fish do you have in that tank?