Long tentacle plate corals are usually rated very difficult to keep, simply because they are so sensitive to damage. Most are doomed from the start, since shipping is so hard on corals. They need to be placed on the bottom of the tank, on a fine sandbed. The least bit of damage can and usually causes the coral to perish. The skeletons of plate corals (almost every LPS actually) is very sharp, plates more so, and any forces apply to the flesh, other then waves, the skeleton will cut through the flesh like a knife. Accidently knock a piece of LR onto the coral, its done. Even frag plugs and large snails can be threatening to the LTP if dropped. If the clowns host the LTP (I had a pair that did), it's a death sentence for the coral as the clows are rough to their hosts.
Your lighting is more then enough. Unlike most other LPS, like frogspawns which get 95% of their energy from photosynthensis, LTP's only get around 80%, so they need to be feed regularly as usually. The do get large. A typical medium specimen will open out to 6"-8" in diameter and a huge one can be the size of a dinner plate. They are also capable of basic locomotion and will adjust themselves to get the light they like. Open sand room is often a premium in smaller tanks. If aquascaped correctly a 30gal will be big enough. I had mine in a 29gal.
