Whilst Im not saying GBR's arent sensitive, they are (sometimes very), the 'they need a mature tank' doesnt necessarily mean you cant have them. The recommendation for mature tanks comes from, IMO, the need to ensure that the tank is fully cycled, absolutely no traces of ammonia or nitrIte and very low nitrAtes, as GBR's are sensitive to nitrAtes, the recommendation to keep them under 20ppm. They are no use to cycle a tank or suitable for a tank thats just at the end of its cycle. However, I really dont believe that leaving the tank for 6 months before adding them is necessary (again this is my opinion) just ensure that the tank wont go into a mini cycle by adding them (ie at the end of a fish in cycle).
This is based purely on my own experience with these fish, I got 3 GBR's and placed them into a tank that had just finished a fishless cycle (which took approx 4 wks), no problems at all, a few months later, I transferred them to another tank (larger) that again had just finished a fishless cycle - never had a problem with them, they thrived. Not only that my water is hard and my ph is 7.8, above the recommended ideal for these fish.
Of course, bolivian rams are hardier (so they say) but Im just saying this over cautious must be mature tank, is a little OTT, so long as the tank has been properly cycled I see/had no problems with them.