Sure you can use T5's on a FW tank. Advanced lighting is usually a good thing especially if it's going to be a planted tank. I go all out using HQI lighting on my 90 gal planted tank.
A couple of areas to look into -
You don't state if you want this to be a planted tank or not. Brackish water will limit the plants you can keep.
The intense lighting of your T5s may cause a lot of unwanted algae growth, especially if you don't control nitrates and phosphates. In planted tanks, this is often done by using dense planting. In other words, lots of plants covering most of the bottom.
The color temp of the bulbs depends upon the effect you want in the tank. I use a low color temp bulbs, only 6500K, on my planted tank. This gives a high PAR value, and makes the tank look like a sunlit pool. The downside is that the light is very yellow, so plants look a lighter shade of green.
For a more balanced color, go with something about 10000K. Anything much above that in a FW tank tends to look too blue, and I wouldn't recommend that unless you want some sort of special lighting effect.
You can also use a combo of bulbs. Something like a 6500K bulb and a 10000K bulb could work well.
