Clowns do best in pairs. You can get a mated pair or just get juveniles and they will typically form a pair (one will become dominant and turn female). With a 90 gallon tank you've got room for whatever species you want, but keep in mind that certain species like Tomatos and Maroons will get aggressive as they mature. Captive bread clowns are commonly available and I recommend going this route whenever possible.
Hippo tangs get large and require a lot of swimming room. IMO a 90 gallon tank is borderline for this species in the long term. IMO they are also not as hardy or useful for algae grazing as other tang species. That said, if you really want one, do some searches here and read-up on their dietary needs, etc. Consider smaller, more useful/hardy species like a Kole, Tomini, Yellow, Scopas, etc. Make sure to research any species you're considering as many tangs can be quite aggressive.
Of the hosting anemones, it's generally agreed that Bubble Tips (BTAs) are the easiest to care for and captive propagated specimens are readily available. That said, an anemone is not a great animal to start into reefing with. They require strong lighting and good water quality to survive long term. There are many possibilities for a surrogate host that are easier to care for. An excellent book on this topic is Joyce Wilkerson's book on Clownfish.
As for stars and crabs (other than mini/brittle stars that hitchhike on in on live rock) I've generally avoided adding either in my tanks so I'll leave those recommendations to others. If you do some searches here I'm sure you'll find plenty of discussion on them.
Don't mean to rain on your parade or discourage you - just want to give you the whole story before you jump in.
HTH