If you want to cover all the bases I think you would want at the least a 250 watt. Even better IMO
would be a 400 watt suspended higher over the tank. 400 watts would give you plenty of light penetration and let you keep the lamp higher over the water helping with heat issues. just my thoughts. I use a 175 watt over my 20 gallon tank, it is 12 inches deep.
Soft corals can produce toxins (chemical warfare) so some people don't mix softies and LPS
. Running GAC
in a sump or canister can keep these toxins in check and you usually won't have any problems. Other things to consider are that some LPS
corals can send out stinging sweeper tentacles that will damage nearby corals.
I recommend you pick up some good reef books that you can reference what corals you buy. Look for some of the books in this thread... http://aquariumadvice.com/viewtopic.php?t=60483