First thing I would do, is check the halides for wattage. It should be either on the base of the bulbs, or on the fixture itself. Single ended, screw type bulbs, are usually either 175-250-400w. If they are 400's, unless you want a SPS
dominated reef, it's probably going to be too much light. 175-250's will still be a lot of light, but would work on your 75g.
Next thing I would do, is only order the compact bulbs, so you can see if it's the bulbs, or the ballasts. If you put new bulbs and the lights still don't work, it's time to make a decision, IMO
With what 2 halides, 2 compacts, and 2 ballasts are going to cost, you could return the compacts, sell the fixture, maybe add a little money, and get new LED
's. With the fixture you have, you are probably looking at around $200 yearly, to replace bulbs, with mid-price range bulbs. With LED
's, they run cooler, are MUCH less costly on your power bill, and have an approximate bulb life of 10 years!
Some of the Cheaper LED
systems, are right around $180-200 apiece. 2 of these would light your tank up. The ones I am talking about are Taotronics dimmable 120w. I don't have these, but many members do, if this is something you are interested in, put up a thread asking for reviews of these lights.
I am just offering this advice because, in the end, this would be much more cost effective for you.
If the compacts work, and it's not the ballasts, and the halides aren't 400's, the light should work fine for you, you will just have to deal with yearly cost for replacement bulbs.
The reason I bring this to your attention is, because even at $18 apiece for 3' T5 bulbs, which is pretty cheap, my yearly expense for bulbs is $360+shipping. I am also looking at getting some LED
Oh, and the ballasts ate usually in the hood, unless the fixture came with a big metal box, if so, they are in there. Also, if it does have the big metal box, that usually means that this is an older fixture, and won't be very energy efficient.