I have seen my GBR pair up as soon as 3 months lately.
They may start to bond earlier but hard to see.
I do believe the more fry there are in the tank, the more a pair may strengthen trying to distinguish themselves from the group.
Last time with my yellow guys when 1 pair formed and laid eggs, 4 other pairs were starting to bond and separate themselves also!.
I know everyone says keeping the Bolivians are easier then the GBR, which seems true ,but without any fact to cite,I believe breeding and raising the two would be equally difficult IMO.
You should confidently expect as good results with your GBR as you have had with the Bolivians!
Quickie sexing your Bolivians is biggest are males right now!
Don't discount the smaller,look for the difference from the largest if they are not bonding.
Try to note the difference in the bonding pair for future sexing reference.
Good luck and looking forward to pics of the GBR
!