Unfortunately you can't change crawl settings for Google in a robots.txt file, they just ignore it. The best way to rate limit them is using custom Crawl settings in Google Webmaster Tools. (look under Site configuration > Settings)
You also might want to consider using your loadbalancer to direct Google (and other search engines) to a "condomised" group of servers (app, db, cache, search) thereby ensuring your users arent inadvertantly hit by perfomance issues caused by over zealous bot crawling.
