I have a 4 bulb t5 over a 75 and have sps. Does it grow like crazy? No, but it stays alive and does grow! As long as you can keep your tank stable, you might be able to keep a nice selection of corals going.
ORA Hawkins Blue half way down in the tank under a 4 bulb t5
I'm no expert on t5s but my understanding is that the reflectors can make a huge difference. If you have individual reflectors for each bulb then you get much better growth, this is what I have been told. Unfortunately I'm guessing you have only one reflector; still though you should try out some small frags of different types of coral to see how they do. Softies are the best anyways. :wink:
I do agree on the reflector's etc.. Not all T5 fixtures are the same. As far as LPS though you should be fine as long as your tank is doing well parameters wise. I kept a lot of lps under a 4 bulb T12's back in the day(75g), and everything was fine. Corals would stretch out a bit but all lived and grew, even a bubble tip anemone. You should be good, just start out slow. Maybe buy a small piece of something and keep an eye on it a few weeks and see how it does.
I would say that husbandry would be the limiting factor in what you are able to keep, not lighting. If you use high quality bulbs, ATI for example, you should be able to keep just about anything that you want.
I typically run less light on my tanks than most other people, I run a 7 bulb T5 light on my 150g tank. Even with acropora towards the bottom of the tank, the growth is above average.
You are limited in bulb option with 18w bulbs. I believe the smallest bulbs that ATI makes is 24w. If you can find good bulbs and have good reflectors you should be able to keep more than softies. I'd even give sps a try just keep it at the top.