Well, you might want to take the time to figure out just how much light you could possibly lose by having too large of a fixture. It may be smarter to purchase the proper size fixture for the aquarium with slightly less juice as far as watts go. saves on electricity bill as well as heat.
As far as the star fish, and other small critters... that's normal as far as i've noticed. In my setup i'll notice a real cool lookin little critter, and then never see him again for 3-4 weeks, so that's absolutely normal

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Andy
My 29 Gallon Reef! My DIY LED Lights Current Status - Camping - Back 7/21/08
I'm still around people, just on a sort of "summer-time leave"

Feel free to PM or e-mail me for any needs!