I'd suggest using only the 4 x 54w fixture. Put in two 6500K bulbs and two 10000K bulbs, but only run TWO bulbs at a time (not all four). If your fiancé ever wants to get injected CO2, then she can run all four bulbs a few hours in the middle of the day (called a noon burst) while running only two bulbs the rest of the time.
To use the bulbs evenly, you can put each bank of bulbs on separate timers and rotate which set of bulbs are burning.
For example (notice no overlap, bulbs run same number of hours each day)
Bank 1 (inside bank of bulbs) -- from 10am to 2pm
Bank 2 (outside bank of bulbs) -- from 2pm to 6pm
With a 2 hour noon burst, here's how it would go (notice all four bulbs running from 1pm to 3pm)
Bank 1 (inside bank of bulbs) -- from 10am to 3pm
Bank 2 (outside bank of bulbs) -- from 1pm to 6 pm
There are many other options from which you can choose; however, since I have a 75g and a 90g with a 4 x 54w T5HO fixture, I am very familiar with this setup. I only run all four bulbs when I'm running injected CO2; otherwise, I'm just begging for algae problems.
You can probably run the bulbs longer than 8 hours a day, but that's a good number to start with. After the tank's been running like that for the first few months, if all is going well (no algae problems), you can start increasing the photoperiod slowly (maybe add 30 minutes every week or two) until you get what you want. I have a very long photoperiod of 12 hours, but most people prefer less.