http://en.memory-alpha.org/wiki/The_Infinite_Vulcan_(episode)#Background_Information has a lot of interesting background information. Apparently, the idea of plants come from Gene. Koenig claims he thought of cloning a force of peace-keepers. It doesn't say who came up with the idea of giants, as far as I can find. All it says about giants is that Gene and D.C. thought it would work well in animation, which is of course true. Maybe it's discussed further in one of the cited sources.