Well Donald is still doing just fine. His eating and growth has dropped off a lot, I only feed him once a week or maybe twice when his belly goes down.
I added white sand, I didn't have it before so could keep the system clean to maximize growth and not stunt him, but now his growth slowed down he can have sand. I did add another powerhead to help keep the sand clean from all the pleco poo! Donald's color is much lighter now.
I grew out a red tail gourami in a 40b for a couple months to try as a tankmate and put him in a month (ish) ago. Donald had taken a few chucks of fin out, but I think the RTGG is tough enough that Donald's exploratory bites don't go past his armor of scales, like the pleco. I mean I am sure D could take a chunk out if he wanted, but he seems to just nip fins of the RTGG and pleco. The RTGG might eventually out grow the 120, if he starts to look cramped I will just give him away, no worries.