Your overflow should be protected with plastic mesh on top to stop all your future creatures from getting in there.
Sea hares are supposed to be able to "ink" like a squid, and probably not the best creature to have in your tank if/when they die (they don't live more than 1 year I think) but I had one in my 50g to clean up my hair algae and when he passed on I didn't have any ill effects on the tank, so your much larger tank would not be in much peril I would say.
You dont have much for corals which is good because they can knock frags around pretty good.
If you have 3" hair algae, a sea hare is a good choice. You will definately need snails as well though, add 20-30 at a time, you dont want to add a ton at first and have them die off later due to lack of food, but you will probably need 100+ when you get your full reef action on!
Oh, and you might think they are ugly now, but they grow on you. So ugly they are cute!
