wow, no kidding on the above "no"s RTC = 1000g+ (really a pond fish or commercial sized aquaria fish) Aro =(depending on the type) 300g+ ID Shark = 1000g+ (again really a pond fish) BGK alone in a 75g = maybe. With an Oscar = no.
Several different types of plecos will work, but certainly not a royal or similar. The trouble is most of the smaller ones seem to be more expensive. Whereas a common for a couple of bucks will likely hit the 16-18" adult range which would be uncomfortable in a 75. My L83/L165 gets even bigger but can be found pretty easily for sub 10$.
I like his idea of the birchir, seems to be a nice size fit.