Ice, as I stated earlier, flourescent lighting can be very complicated and is highly engineered. Yes, distance to the metal is critical so if you switch from a T12 (1 1/2" diameter tube) to a T8 (1" diameter tube) you may have exceeded the 1/2" limitation, unless you changed out the lamp bases (tombstones).
And, the list goes on and on. Here is an excellent website to learn all you ever wanted to know (and more) about the topic. It's free and available to anyone, but you do need to register to take the course. http://www.advance.philips.com/university/
Good Luck from Columbiana, Alabama
Maurice Turgeon, http://thesemi-retiredelectrician.com