Glass catfish grow to 4-6 inches and you need a school of at least 6; these are fish that really can pine away and die if left alone. So no space for those, I'm afraid.
Tbh, before looking into additions, it is always a good idea to review the current stocking first. In your case, there is a pair of corys who would get a lot more entertaining if they had a couple of mates. I would start there and count another couple of inches in for more mates.
Then there is the plec. Do you know what species he is? How big is he going to grow and how soon? Have you got plans for what to do if he outgrows the tank (if he is a common or sailfin he will, and soon). You may have 8 inches left at the moment, but how long is that going to last? (also, plecs count for more than their inches, due to their messiness).
If you want another addition, it had better not be too large or messy. My inclination would be to leave it. But maybe you could fit a False flying fox in, if you have a contingency plan for the plec.
