i have a couple kuhlis in ever one of my tanks and i may see one every 3 months while im vacuuming the gravel. i would say that you should expect to never see them but it sounds like other people have better luck with them then me.
If kuhlis are the only bottom feeder, a 28g should be fine. I'd keep at least 6 so they don't hide all the time. The more plants & hiding places they have will make them feel secure so they'll be out more.