a GBR will be fine in a 29g on its own. They can be kept individually in tanks even smaller. If aggression is your concern, then add the ram after all of the passive fish. I assume this will be last unless you're planning serpae tetras or the like. The idea is that he will be coming in to a tank with fish all ready there, and have to carve out his own territory with them allready having established patterns in the tank. He'll be a little more distracted with the entirety of a new habitat, rather than having these fish added after to be individualy examined. There are a few other ways of interpreting this, but basically it's the same old home field advantage concept.