From what I've read ... 65 & 55's put out almost the same amount of light in tests, so it doesn't much matter which one you use (although you will pay a bit less in electricity with the 55W).
Also, modern electronic ballasts are load sensing and will fire up a varielty of bulbs so you can ususally interchange 55 & 65 in the same fixture (read the ratings on the ballast to be sure - as long as the total watts of you bulbs is less than the max rated and the ballast is auto load sensing electronic, it should work.).