Probably picking up a URI electronic ballast at work this week(got a good deal on one). Anyway, it says it can operate up to 4 bulbs that are no more than 16ft in length. I already have two 6ft tubes but my tank is 7ft, so can I add two 2 foot tubes or will the ballast freak out if it's running different sized tubes? Wouuld I be better running four 3ft or four 4ft tubes?