I am not sure I agree about what the above poster said about glass fish, but first are the glass catfish? I keep 2 of these and it is not concrete that they need a group, just another. I would however say you should get one more because they are not usually good in odd numbers as one gets left out. Maybe this is not the case with yours?
I'd recommend bottom feeders of some kind. I think it's important to get a god balance between top/middle/bottom dwellers.
Is there anything you have had your eye on other than the 3 spot?
HTH