medium-sized schooling fish for 125g?

Some of the larger rainbows should be okay. My turquoise are about 3.5 inches right now, fast swimmers, good schoolers. Very nice colors, as well.
 
Try some rosy barbs or some of the larger rainbows.
 
tinfoil barbs are an interesting idea; however i am concerned that they would get too big to adequately house both a school of them and other fish i enjoy.

rainbowfish are a possiblity...i've never kept them before. do you think they would irritate the slow-moving, mellow gouramis?
 
I had gouramies and rainbows together for a while, but moved the gouramies because the blue tetras annoyed the heck out of them. The gouramies actually shared their hang outs with the rainbows, but would chase out the shark and algae eater.

Rainbows are really easy to take care of--they eat anything, don't harrass anyone (least in my tank), and are really pretty.
 
Rainbows are awesome fish. They are very streamlined and move through the water like a torpedo. They are fast and people have used them as dither fish. If I ever get around to setting up a community tank, I think Rainbows would be very high on my list of possibilities.
 
AquariaCentral.com