I also wouldn't do the dsb. No need taking up all that space inside the aquarium which takes up the room the fish can swim. You also have to look at the sand bed when viewing the tank.... Unless you plan on keeping critters in there that need a deep substrate.... No need going more then .5-1 inches deep
Make sure play sand is acceptable for aquarium use. Some work great and is the same stuff that we pay 10 times for at the aquarium store. Others are full of phosphate (I think that's what it was) and will cause all kinds of problems as the days go by.
Nothing wrong with three 300w heaters. Set on at the operating temp... The second one 4-5 degrees lower and the third one several degrees lower. You do not want all three set at the same temp where they turn on/off constantly together. Have one main one and then one or two back up heaters.
250gph skimmer? Not sure what you mean by that. A skimmer that can only process 250gph is small. I would get a good one and not a cheap one
Uv... Not a necessity and AT&T hat size will only affect algae growth. Probably only slightly.... If you want to kill other stuff in the water get a much larger one and preferably not a internal one
Damsels can get VERY mean as they get old.
Tangs - certain ones might be ok however most get large and need a lot of room.
Queen angel - NO... Your tank is way to smal.... My 10ft tank seems to be small for my queen angel... Beautiful fish but a attitude that matches its looks. Dwarf angel or a lg angel that doesn't get as big or have the attitude of the queen.
Clowns.... Like stated usually you only want a pair of them