Sorry to hear about your chromis. Unfortunately some fish get sick without us being aware of it until, as you found out, there stuck to a powerheads intake.
In a 90 gallon tank you can have a yellow tang. In the fish forum, can't think of it's exact name, there is a sticky, which is a permenant post, of the recommended size of your tank to suit most of the popular tangs in the hobby. Worth checking out, so you don't have critics later.
Flooded the floor
Any honest reefer will tell you the same has happened to them. I put my buckets in the laundry sink now, as I have done the same trick as you twice
This way if it overflows, it's not a problem, except when you go out and forget it's on
As for your flow, since you are planning sps i'd go the k4's. Softies handle flow quite well, not to direct. You can direct the flow, so think about how you aquascape and placement of corals. Softies and LPS lower in the tank, SPS up high.
I have no experience with MH, but I love T5's. I don't have heat issues, and you can have more bulbs of different colours. However, with more bulbs and a hood, make sure you put in some fans to blow out excess heat
Oh yeah, hold off on the mandarin until your tank ages and settles in. Blennies are great and so are corris wrasses, here's a picture of mine, about 2 years old, marvellous fish.
Cheers
Chris