i think a kole would be fine.

You are correct. Kole tangs are an exception. They are film algae eaters and spend all their time sucking the film off the glass and they stay small longer.
Yellows, blacks, purples and tomini tangs still require the longer tanks because they are all active swimmers.

kole tangs were one of the ones i have been looking into. tominis look pretty cool too. i am planning for a central island configuration for the rock (diy one piece rock is the plan).

I think if you go with "smaller" tangs you will be ok. Have a look at this thread it has been done before.

http://www.canreef.com/vbulletin/showthread.php?t=49882

I think you can easily house either a Kole or a Tomini tang in your cube provided you buy a small specimen to start with and you create a central rock formation that the fully grown tang can easily swim around in circles. That means you won't be able to fit as much rock as is typical in many reefs. Keep in mind also that your corals will grow out from this rock, and encroach on the tang's swimming space. I suggest you pick a small specimen because it will adjust to the abnormal swimming area much better than a medium or large sized specimen that may be quite used to swimming either the ocean or a longer typically shaped aquarium. The Tomini typically will grow faster than the Kole, but personally I like the looks and personality of the Tomini better. I would also suggest you add this fish last, and after the rock has sufficiently aged (especially if you are using diy rock), say 8-12 months.