Check out the book Clownfishes:A guide to their captive care, breeding, & natural history by Joyce Wilkerson. Its a great book and I recommend it to everyone planning on keeping clownfish.
As for your tank... Sometimes you can get away with multiple types of clowns if the species are different enough in appearence and if you provide enough "territories" for them.
My other opinions:
1. Buy captive bred. Clownfish are one of the few marine fish that frequently available as captive bred. You will get higher quality, higher disease resistance, and friendlier fish. It also helps support captive breeding research and takes pressure off wild populations.
2. If you are planning on pairs - you will need to get a very very young (1/4" size) or a true mated pair. The later is most likely wild caught unless you personally know the breeder and he is doing you a favor. If you find young ones you will need to start small as I said and buy 2-4 to start with. As they mature the dominant one will become the female and the next will become the male - the others will remain juveniles and get beat up by the female...you will eventually end up with just the pair or you can move the others into a separate tank and allow them to pair off as well.
3. use hairy mushrooms and other soft corals as surrogates instead of anemones - again they are much easier to care for and you can find captive propagated shrooms etc.
here are some stats regarding anemone survival in home aquaria (taken from the book above):
*46% of anemones kept by hobbyist with <2yrs experience were dead after an average of 3.5 months
*40% anemones kept by hobbyists with 2-5 yrs exp. were dead by 11.5 months
*27% anemones.........hobbyist w/ 5+ yrs exp. were dead after 14 months
some biologists believe that anemones in the wild can live up to 100 yrs ...some longer. Anemones that are taken from the wild rarely survive and worse yet fail to provide settling habitat for the wild fish fry(clowns, damsels, etc) that need them for protection.
Sorry...I'm off my soap box, didn't mean be preachy just trying to spread the word.