I understand that some birds are more "trainable" than others so in curious what I should be focussing on with Gus (our GCC) and what we should just avoid if its beyond his capability.
I think you need to give Gus more credit . Birds are smart critters; I did recall training with the family budgie when I was 10, and the little guy picked it up right away. The only difficulty I find with training birds, especially flighted ones, is that they'll decide they just rather do something else! So it's important to keep up his focus and motivation.